Invertebrate Anatomy OnLine
Homarus americanus ©
American Lobster
with notes on crayfish
Copyright 2001 by
Richard Fox
Lander University
            This is one of many exercises available from Invertebrate Anatomy OnLine , an Internet laboratory manual for courses in Invertebrate Zoology. Additional exercises can be accessed by clicking on the links to the left. A glossary and chapters on supplies and laboratory techniques are also available. Terminology and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated figure callouts refer to figures in the textbook. Callouts that are not hyphenated refer to figures embedded in the exercise. The glossary includes terms from this textbook as well as the laboratory exercises. 
Arthropoda P, Mandibulata, Crustacea sP, Eucrustacea, Thoracopoda, Phyllopodomorpha, Ostraca, Malacostraca C, Eumalacostraca, Caridoida, Decapoda O, Dendrobranchiata sO, Astacidea iO, Nephropoidea SF, Nephropidae F, (Fig 16-15, 19-67, 19-90)
Arthropoda P
            Arthropoda, by far the largest and most diverse animal taxon, includes chelicerates, insects, myriapods, and crustaceans as well as many extinct taxa such as Trilobitomorpha. The segmented body primitively bears a pair of jointed appendages on each segment. The epidermis secretes a complex cuticular exoskeleton which must be molted to permit increase in size. Extant arthropods exhibit regional specialization in the structure and function of segments and appendages but the ancestor probably had similar appendages on all segments. The body is typically divided into a head and trunk, of which the trunk is often further divided into thorax and abdomen.   
            The gut consists of foregut, midgut, and hindgut and extends the length of the body from anterior mouth to posterior anus. Foregut and hindgut are epidermal invaginations, being derived from the embryonic stomodeum and proctodeum respectively, and are lined by cuticle, as are all epidermal surfaces of arthropods. The midgut is endodermal and is responsible for most enzyme secretion, hydrolysis, and absorption.
            The coelom is reduced to small spaces associated with the gonads and kidney. The functional body cavity is a spacious hemocoel divided by a horizontal diaphragm into a dorsal pericardial sinus and a much larger perivisceral sinus. Sometimes there is a small ventral perineural sinus surrounding the ventral nerve cord. 
            The hemal system includes a dorsal, contractile, tubular, ostiate heart that pumps blood to the hemocoel. Excretory organs vary with taxon and include Malpighian tubules, saccate nephridia, and nephrocytes. Respiratory organs also vary with taxon and include many types of gills, book lungs, and tracheae.  
            The nervous system consists of a dorsal, anterior brain of two or three pairs of ganglia, circumenteric connectives, and a paired ventral nerve cord with segmental ganglia and segmental peripheral nerves. Various degrees of condensation and cephalization are found in different taxa.
            Development is derived with centrolecithal eggs and superficial cleavage. There is frequently a larva although development is direct in many. Juveniles pass through a series of instars separated by molts until reaching the adult size and reproductive condition. At this time molting and growth may cease or continue, depending on taxon.
            Mandibulata is the sister taxon of Chelicerata and in contrast has antennae on the first head segment, mandibles on the third, and maxillae on the fourth. The brain is a syncerebrum with three pairs of ganglia rather than the two of chelicerates. The ancestral mandibulate probably had biramous appendages and a J-shaped gut, posterior-facing mouth, and a ventral food groove. The two highest level mandibulate taxa are Crustacea and Tracheata.
Crustacea sP
            Crustacea is the sister taxon of Tracheata and is different in having antennae on the second head segment resulting in a total of 2 pairs, which is unique. The original crustacean appendages were biramous but uniramous limbs are common in derived taxa. The original tagmata were head but this has been replaced by head, thorax, and abdomen or cephalothorax and abdomen in many taxa. Excretion is via one, sometimes two, pairs of saccate nephridia and respiration is accomplished by a wide variety of gills, sometimes by the body surface. The nauplius is the earliest hatching stage and the naupliar eye consists of three or four median ocelli.
            Eucrustacea includes all Recent crustaceans except the remipedes. The taxon is characterized by a primary tagmosis consisting of heat, thorax, and abdomen although the derived condition of cephalothorax and abdomen is more common. Eight is the maximum number of thoracic segments.
            In the ancestral thoracopod the thoracic appendages were turgor appendages used for suspension feeding in conjunction with a ventral food groove. Such appendages and feeding persist in several Recent taxa but have been modified in many others.
            The compound eyes are stalked primitively although derived sessile eyes occur in many taxa.
Malacostraca C
            Malacostraca includes most of the large and familiar crustaceans such as crabs, shrimps, lobsters, crayfish, isopods, and amphipods. Primitively the trunk consists of 15 segments, eight in the thorax and seven in the abdomen but in most Recent species the abdomen has only six segments (Fig 19-19). The female gonopore is on the eighth thoracic segment and the male on the sixth.
Decapoda O
            The largest and most familiar crustaceans belong to Decapoda. The 10,000 species of crabs, shrimps, crayfishes, lobsters, and their relatives are decapods. The first three segments of the decapod thorax are fused with the head to form a cephalothorax and their appendages are maxillipeds. The remaining five pairs of thoracic appendages bear simple or chelate walking legs. The resulting ten legs accounts for the name “decapod”. A large carapace extends posteriorly from the head and is fused dorsally with all eight thoracic segments. Laterally the overhang of the carapace encloses the branchial chamber with the gills. The most primitive decapods (shrimps, lobsters, and crayfishes) have well developed abdomens whereas the most derived taxa (true crabs in Brachyura) have reduced, almost vestigial, abdomens (Fig 19-24).
Laboratory Specimens
            The crustaceans in Astacidea iO are excellent examples of the primitive crustacean condition. In this group are the clawed lobsters in Nephropoidea SF and the freshwater crayfishes in Astacoidea SF, either of which can be used for this study. 
            This exercise is written specifically for the American lobster, Homarus americanus , but can also be used with any genus of freshwater crayfish or any other clawed lobster such as Nephrops. These animals differ chiefly in minor anatomical details such as the number of abdominal appendages and gills which should cause no confusion. The obvious advantage of the lobster for this study is its large size. Its availability alive from local supermarkets is also an advantage. The disadvantage is the expense.
External Anatomy
            Place a living or preserved lobster or crayfish on a dissecting pan of appropriate size and take it to your bench for study. The claws of active lobsters must be restrained with strong elastic bands. 
            Lobsters can be relaxed with magnesium chloride. A 20 cm (body length) lobster will succumb to magnesium chloride in about 2 hours. It will be quiescent in about 1 hour. After one hour it can be removed from the relaxant and handled. If returned to seawater it may recover and become active. If kept out of seawater during the exercise it will soon die. 
            Lobsters and crayfish have bodies similar to the ancestral "caridoid facies" of the presumed ancestral crustacean. Such a body is essentially shrimp-like in that it is elongate and nearly cylindrical in cross section. The abdomen is well developed and its segments and appendages are readily apparent. Note the bilateral symmetry
Body Wall
            The decapod body wall is reduced to just two layers, the exoskeleton and the epidermis that secretes it. No layers of muscle are present because the circular and longitudinal body wall muscles have become specialized individual muscles and no longer form continuous unspecialized layers as they do in the worm-like ancestors of the arthropods. Continuous layers of circular and longitudinal muscles would be useless under a solid and immovable exoskeleton. Connective tissue is absent because the exoskeleton itself provides strength, structural support, and protection. No peritoneum is present because the body cavity is a hemocoel, not a coelom. The exoskeleton is secreted by the epidermis and is molted periodically to allow the animal to increase in size. 
            A complicated musculature, derived from the circular and longitudinal muscles of the ancestors, originates and inserts on the inner surface of the exoskeleton and moves its many parts. The exoskeleton between adjacent regions is thin and flexible to permit motion. There is also a complicated "endoskeleton" composed of internal processes, the apodemes, from the inner surface of the exoskeleton. The gills and the anterior and posterior regions of the gut are covered with a very thin exoskeleton. 
            Many parts of the exoskeleton bear small, articulated, movable bristles, or setae. These can be seen at many places on the body, as for example on the cutting edge of the smaller claw. Thicker articulated processes from the exoskeleton are spines. Processes that are simply outgrowths of the exoskeleton and are not articulated are usually called teeth.
            The connective tissue compartment is well developed and occupies most of the interior of the animal. Most of it is a blood space, the hemocoel.
            The body is composed of a linear series of segments. In malacostracans there are 19 segments but you cannot see or count them all. Each one bears a pair of jointed appendages, which can be seen and counted. 
            In the ancestral crustacean all segments were identical (homonomous), or nearly so, as were their appendages. In derived crustaceans the segments and their appendages are specialized for various purposes and to a large extent no longer resemble each other closely (heteronomous). 
            Groups of adjacent segments and their appendages tend to function similarly and together accomplish certain specialized tasks. This results in a regionalization of the body into tagmata. In crustaceans there are typically three tagmata; the head, thorax, and abdomen but secondary tagmosis frequently modifies this plan. 
            The crustacean head is always composed of five segments but the thorax and abdomen are variable. In Malacostraca, however, there are always eight segments in the thorax and almost always six in the abdomen. The anteriormost tagma is the head (Fig 1). The middle region is the thorax and the posterior region is the abdomen. In decapods, as in many crustaceans, the head and anterior part of the thorax are combined to form a new tagma, the cephalothorax leaving the remaining five thorax segments to form another new tagma, the pleon. It is customary to refer to the combined head and all thoracic segments as the cephalothorax when discussing decapods but that practice is not applied uniformly to other crustacean taxa and is not followed here. 
Figure 1. A dorsal view of an American lobster, Homarus americanus. Adapted from Herrick, 1909. crab32L.gif
Figure 1 
            The cephalothorax is covered dorsally and laterally, but not ventrally, by a double sheet of exoskeleton called the carapace (Fig 1, 14, 19-1). Although you cannot tell it by looking at the animal, the carapace is an outgrowth of the exoskeleton of the most posterior head segment. It grows posteriorly to cover and protect the thoracic segments. It is a fold of the body wall and as such consists of two complete layers of the wall. The outer wall of the carapace is a thick sclerotized exoskeleton and is hard and strong but the inner wall has only a thin exoskeleton and is transparent and flexible (Fig 14). The carapace forms a shell over the back and sides of the thorax with which it is attached dorsally. The lateral extensions of the carapace, known as branchiostegites, enclose large lateral branchial chambers which house the gills (Fig 14, 19-36, 19-3A). 
Head and Cephalothorax
            The five segments of the head and first three thoracic segments are fused together and cannot be distinguished. Together these eight segments are the cephalothorax. Each cephalothoracic segment bears a pair of appendages. No sign of segmentation can be seen on any surface of the head. The carapace covers more than just the cephalothorax and the two are not synonymous. Dorsally and laterally the entire thorax is covered by the carapace. 
            A conspicuous transverse cervical groove divides the carapace into an anterior third, which is roughly the cephalothorax, and posterior two thirds, which is the rest of the thorax (pereon) (Fig 1).   
            Anteriorly the head bears a conspicuous anterior, median process, the rostrum. The orbits are a pair of semicircular notches, or sinuses, in the carapace lateral to the base of the rostrum. Each orbit contains aneyestalk with a compound eye at its distal end. The black, multifaceted cornea of the eye covers the entire circumference of the end of the stalk except medially. The anterior end of the head is the acron but it is fused with the head and cannot be distinguished from it. 
            The thorax is composed of eight segments, called thoracomeres, and as we have seen, all eight are hidden beneath the carapace, when viewed dorsally or laterally. Each thoracic segment bears a pair of appendages known by the general term thoracopod
            The anterior thorax, consisting of three segments, is fused with the head to form the cephalothorax, as described above.  The posterior five segments remain independent of each other and of the head. The posterior thorax, composed of these five segments, is the pereon and its segments are known specifically as pereomeres (= pereonites), which are a type of thoracomere. The pereon is not part of the cephalothorax even though it is covered by the carapace.
            Under the carapace, the posterior thorax, or pereon, is segmented. This segmentation is apparent ventrally where he pereon is not covered by carapace.
            The abdomen (Fig 1, 19-1) of primitive decapods is well developed with clearly visible segments and powerful longitudinal muscles. It is this abdominal musculature that is largely responsible for the great popularity of lobsters and crayfish at the table. 
            The abdomen is also called the pleon and its segments are pleomeres (= pleonites). Count the abdominal segments. All six are clearly visible and none is fused with each other or with the thorax, nor are they covered by the carapace. 
            The posterior end of the body is the telson, which is not a segment (Fig 1, 19-1). The anus is located on the ventral side of the telson.
Abdominal Exoskeleton
            The exoskeleton of the abdominal segments of the lobster approximates the typical ancestral condition. Primitively, each body segment is enclosed in skeletal ring of four articulated exoskeletal plates, or sclerites, that form a complete circle around the segment (Fig 16-1B). The dorsal sclerite is the tergite, the ventral one the sternite, and on the sides are two lateral pleurites
            In astacideans (lobsters and crayfish) the tergites and pleurites are fused together to form a hard arch of exoskeleton covering the dorsal and lateral aspects of the segment. The pleurites extend ventrally past the body as side plates, or epimera, which together form a shallow ventral space below the abdomen. 
            The sternites cover most of the ventral surface of the abdomen and the pleurites cover the lateral parts of it. Most of a sternite is thinner and more flexible than the tergites and pleura. It is transparent and the abdominal musculature and ventral nerve cord can be seen through it. The posterior margin of each sternite however is very thick and heavy and forms a reinforcing arch across the venter from one pleurite to the other. The appendages articulate with the pleurites at the ends of this sternal arch.     
            Begin the study of appendages by reviewing the morphology of a typical crustacean limb. It is jointed, or composed of articulated sections called articles (not segments), and is primitively biramous. A biramousappendage has a basal article, the protopod that attaches by its proximal end to the body (Fig 1, 2, 19-1). Sometimes the protopod is divided into two articles, the coxa and basis. From its distal end arise two rami, or branches. The rami are an outer, or lateral, exopod and an inner, or medial, endopod (Fig 1, 2). 
            The two rami may be composed of any number of articles depending on their function. They may be similar to each other or different. Sometimes only one ramus is present and the appendage isuniramous. Sometimes there are additional branches of the protopod or of the rami. Any additional branch on the lateral side is an exite and any extra medial branch is an endite. Finally, an exite on the base of the appendage is given the special name of epipod.
            Decapod appendages are easiest to study by beginning at the posterior end and working forward. As you do this, keep in mind that they are named and numbered in the opposite direction, from anterior to posterior. 
Abdominal Appendages
            Each of the six abdominal segments bears a pair of appendages. Most of these are biramous and, more than any other lobster appendage, resemble the ancestral crustacean appendage. 
            The last (posteriormost) pair of abdominal appendages, located on abdominal segment 6, are uropods (Figs 1, 2, 19-1). The uropods have a relatively small protopod and two very large, flat rami. The exopod is biarticulate (composed of two articles). The distal border of each ramus bears a fringe of setae. Spread the rami of the two uropods apart and array them beside the telson. The four rami plus the telson make up the tail fan, which functions as a large paddle (Fig 1). With the fan deployed, flexure of the abdominal muscles moves the fan rapidly forward under the body and results in the generation of a powerful forward jet of water that propels the animal backwards in a characteristic escape response.
            The first five pairs of abdominal appendages (counting from anterior to posterior) are pleopods (= swimmerets)1-5. Pleopods 2-5 are biramous and are similar to each other. Both rami are flat, leaflike phyllopods whose rhythmic movements generate a water current. Female pleopods are better developed than those of males and are used to carry the eggs, which are attached to the fringe of setae around the rami. (The rami of crayfish pleopods are not broad and leaflike. Instead they are narrow and whiplike.)
Figure 2. A lobster uropod. Redrawn from Herrick (1909). Crab33L.gif
Figure 2
Figure 3. Pleopod 3. Redrawn from Herrick (1909). Crab34L.gif
Figure 3
            The second pleopod of male lobsters bears a small endite. The first pleopods are uniramous (Fig 4). In females the first pleopods are small and vestigial but in males they are modified and heavily sclerotized to serve as an intromittent organ to transfer sperm to the female. In males they are referred to as gonopods
Figure 4. Pleopod 1 of female. Modified from Herrick (1909). Crab35L.gif
Figure 4
Branchial Chamber
            The lateral extensions of the carapace, known as branchiostegites (Fig 14, 19-36, 19-3A), enclose the lateral branchial chambers where the gills are located.
"    Lift the ventral edge of the carapace and note that it is attached dorsally to the body but is free laterally. With strong scissors cut away the unattached lateral portion on the left side of the carapace without cutting into the attached portion. Be careful that you do not cut into the body and do not damage the numerous structures in the space below the carapace.  You have removed the left branchiostegite.
            The space thus uncovered is the branchial chamber and it contains the gills. The gills, which are epipods of the thoracic appendages or adjacent pleurites, will be studied later. They are feathery, white, filamentous processes. Keep them moist so they do not dry out. Removal of the branchiostegite exposes the entire length of the thoracic appendages and makes their study easier. 
Thoracic Appendages
            Each thoracic segment bears a pair of appendages but as there are two distinctly different regions of the thorax, two distinctly different types of thoracic appendages characterize those regions. The appendages of the anterior three thoracic segments are maxillipeds and function as auxiliary mouthparts. The appendages of the posterior five thoracic segments (pereomeres) are pereopods and function as walking legs or pincers. Maxillipeds and pereopods are the two types of thoracopods.
            The naming and numbering of thoracic segments and appendages is potentially confusing and requires explanation. The first thoracic segment is thoracomere 1 and it bears maxilliped 1 (which is also thoracopod 1). Thoracomeres 2 and 3 bear maxillipeds 2 and 3 respectively (= thoracopods 2 and 3). The fourth thoracomere is also the first pereomere and its appendage is pereopod 1 (= thoracopod 4) and so forth.
            The five segments of the pereon bear a total of 10 appendages which accounts for the name Decapoda (= ten feet). The ten appendages are pereopods but are usually referred to loosely as "walking legs", whether or not they are used for walking. All pereopods lack the exopod and are uniramous. The endopod is long and narrow. This shape of ramus is referred to as stenopodous in contrast to a broad, flat, leaflike phyllopod.  
            Look at the posteriormost thoracic appendage. It is pereopod 5. Which thoracopod is it? The typical malacostracan thoracopod (including pereopods and maxillipeds alike) is composed of seven articles. The two proximal articles represent the subdivided protopod and the distal five are the five articles of the endopod. 
            Find the seven articles of pereopod 5 (Fig 5, 19-1, 19-20). The proximal article is the coxa. It is wide and short and articulates with the sternite of the last pereomere. Distally it articulates with a short, narrowbasis. The basis joins with the ischium along an oblique articulation. 
            Notice that the ischium appears to be composed of two articles in that it has an oblique groove encircling it near its articulation with the basis. This groove marks the location of the fracture plane where the lobster can deliberately autotomize (auto=self, tome=cut) its limb (Fig 5, 19-36). This plane is specialized for this function and the animal can loose its limb, at this plane only, with minimal trauma or blood loss. 
            The ischium articulates with a long narrow merus. Next there is a short carpus followed by a long propodus. The final article is a sharp, pointed dactyl, or nail.
            A large, white, feathery gill is attached to the pleurite immediately dorsal to the coxa. Gills are associated with all thoracopods except maxilliped 1. Most appendages have more than one gill and they may be attached to the pleurite, coxa, or the articulating membrane between the pleurite and coxa (Fig 14). 
Figure 5. Pereopod 3. Adapted from Herrick, 1909. crab36L.gif
Figure 5
             Pereopods 4 and 5 are almost identical. Pereopod 4 has a large membranous, leaflike epipod on its coxa that is absent from 5. This epipod extends vertically between the gills. Similar epipods are present on the remaining pereopods and on maxilliped 3. 
            Pereopods 1-3 resemble each other in that the propodus and dactyl form a prehensile, or grasping, pincer. The propodus bears a long, fingerlike, distal process against which the dactyl opens and closes (Fig 5, 19-2B). The dactyl is a movable finger and the propodal process is an immovable finger. Such a pincer is known as a chela and appendages bearing them are chelate. Pereopods 4 and five do not have chelae and are " simple". The small chelae of pereopods 2 and 3 are used to transfer food to the mouth (Fig 5). 
            The first pair of pereopods is much larger than any other appendage (Fig 1, 19-2B). They are chelate and, because of the striking size of their chelae, are referred to as chelipeds, and rarely as walking legs. The usual seven articles are present in the chelipeds. The chelae, as expected, are formed of the opposing propodus and dactyl. 
            Notice the strong dimorphism in the two chelipeds. One, the crusher claw, is heavy and massive with low rounded teeth. The other, the cutter claw, is slender and bears sharp, pointed teeth. 
            The crusher has rounded, molarlike teeth and closes relatively slowly but with force sufficient to break the shells of oysters and mussels. The muscle that closes the claw is very large, to the delight of gourmets, and occupies almost the entire interior of the propodus. It is composed entirely of slow fibers. 
            The cutter claw is slimmer and bears sharper teeth. It closes much faster than that of the crusher and can do so many times faster than the human reflex that might try to remove fingers from harm's way. The fibers of its closing muscle are almost entirely fast fibers. Each propodus contains a small opener muscle, in both cases composed of slow fibers, that opens the claw. 
            In about 50% of the population the left claw is the cutter and in the other half it is the crusher. Which side becomes which claw is determined simply by which one happens to get the most use during a critical period early in the life of the individual.
            Notice the variety of articulations in the joints of the chelipeds. Flex and extend each joint to see what kinds of motion its articulation allows. Each joint has an axis on which its two articles rotate with respect to each other. Determine the axis of rotation for each of the six articulations of the cheliped. 
            >1a. Insert a #1 insect pin along the axis of rotation of each joint, or as close to it as the exoskeleton will permit. Are any of the pins parallel to each other, or close to it? How many different planes of motion can you distinguish? Notice how the six different axes together allow the chela to move to almost any position. Might this be a good model for an articulated arm on a Martian rover? Determine the contribution of each articulation to the range of motion of the entire chela by immobilizing each of the articulations. Do any seem to be more important than others?  <       
            The female gonopores are the external openings of the oviducts. They are located on the medial side of the coxa of pereopod 3 (thoracopod 6). The male gonopores are the external openings of the vasa differentia from the testes and are found at the tip of the two short genital papillae on the medial surface of the coxa of pereopod 5 (thoracopod 8). The position of the male and female gonopores is constant throughout Malacostraca. 
            A large, conspicuous, blue, so-called seminal receptacle is located on the ventral surface of the female between the coxae of pereopods 4 and 5. It bears a deep median longitudinal cleft that is slightly expanded anteriorly. The walls of this anterior expansion are flexible and, if pushed with a fine forceps, will open to disclose a recess. The male gonopod (which is the first pleopod and not the genital papilla) is inserted into this flexible pocket where it deposits spermatozoa into the recess. (In crayfish the female receptacle is called the annulus ventralis). 
            The anterior three pairs of thoracic appendages are maxillipeds (Fig 19-2A). These three pairs of thoracopods are biramous, unlike the pereopods, but their endopods still have the usual seven articles. 
            The third maxillipeds are on the third thoracomere immediately anterior to the chelipeds. Each is large and intermediate in shape and size between the pereopods and the mouthparts. Each has a large, stenopodous endopod and a small filamentous exopod (Fig 6). 
            The protopod is divided into a coxa and a basis, as it is in all thoracopods (eg see Fig 5). The endopod arises from the distal end of the basis and consists of the usual five articles of a thoracic endopod (ischium, merus, carpus, propodus, and dactyl). The long heavy ischium bears an impressive row of medial teeth that, along with the mandibles, are the chief means of reducing the size of food particles before ingestion. The smallexopod arises from the distolateral corner of the basis. One function of the third maxilliped is to protect the more delicate appendages anterior to it. 
Figure 6. Maxilliped 3. Redrawn from Herrick (1909). Crab37La.gif
Figure 6
            Hold the third maxillipeds aside and look at the next appendage. It is the second maxilliped (Fig 7). It too is biramous but is much smaller that the third. Its exopod is longer than its endopod. 
Figure 7. Maxilliped 2. Modified from Herrick (1909). Crab38La.gif
Figure 7
            The first maxilliped is the appendage of the first thoracomere (Fig 8). Its exopod resembles those of the other maxillipeds and is long and narrow. Its endopod has two articles and is also long and narrow looking superficially like the exopod (the crayfish endopod is short and inconspicuous). The exopod lies in a groove on its lateral border. Two large wide, thin endites curve over the bulge of the mandible. A long posterior epipodextends posteriorly into the branchial chamber.
Head Appendages
            The remaining five pairs of appendages are those of the five head segments. The posterior three are mouthparts whereas the anterior two are antennae and have a sensory function (Fig 19-2A). 
Figure 8. Maxilliped 1. After Herrick (1909). Crab39La.gif
Figure 8
Figure 9. Maxilla 2. After Herrick (1909). Crab40La.gif
Figure 9
            The second maxilla is the appendage of the fifth head segment and it lies immediately anterior to the first maxilliped (Fig 9). It generates a water current that pumps water out of the anterior end of the branchial chamber. Its basal portion bears four flat, narrow endites, a slender endopod, an epipod, and an exopod. The long flat exopod and epipod form the all-important gill bailer, or scaphognathite (Fig 9, 19-38B), whose motion generates the respiratory current through the branchial chamber.       
            The gill bailer lies beside the carapace and extends anterior to and posterior to the basal part of the second maxilla. The large thin trough-shaped epipod of the first maxilliped extends back toward the branchial chamber (Fig 8). It functions in concert with the gill bailer of the second maxilla. The bailer lies in the trough formed by the epipod of the first maxilliped where it beats to create a current.
            The first maxillae (Fig 10) are small and more delicate than the second. The smallest of the mouthparts, they lie curved tightly against the smooth, hard surface of the mandible. Each has two broad endites and a narrow, larger endopod. The exopod is absent.  
Figure 10. Maxilla 1.  After Herrick (1909). Crab41L.gif
Figure 10
Figure 11. Mandible. Redrawn from Herrick (1909). Crab42L.gif
Figure 11
            The mandibles are the most anterior of the mouthparts. Each is heavily calcified and equipped with powerful muscles (Fig 11). The large basal portion bears a cutting edge on a medial lobe. A three-articled palparches over the cutting edge. The mandible has partial responsibility for shearing small pieces of food from larger ones. It can rotate only slightly on its axis. 
            A single, large, fleshy labrum, or upper lip, attaches to the anterior body wall just dorsal to the mandibles and fills much of the space behind the cutting lobes. The labrum is a fold of the body wall and is not an appendage.
            The remaining two pairs of appendages are the sensory antennae. The biramous second antennae are by far the larger of the two pairs (Fig 12, 19-2). Each arises by a biarticulate protopod consisting of a proximal coxa and a distal basis. The short, wide, phyllopod-like exopod, which is called the antennal scale, arises from the basis. The endopod, which also arises from the basis, has a short thick basal peduncle of three articles and a very long narrow, whiplike flagellum of many articles.   
Figure 12. Antenna 2. The ventral surface and nephridiopore are shown in the inset.   Redrawn from Herrick (1909). Crab43L.gif
Figure 12
            The lower surface of the coxa bears a small circular tubercle with an opening, the nephridiopore, in its center. The nephridiopore is the external opening of the nephridium.
            The first antennae (= antennules), are situated below the eyestalks (Fig 1, 19-2A,B) and are much smaller than the second antennae. Each has a triarticulate basal "protopod" from which arise two slender multiarticulate flagella of nearly equal length (the medial flagellum of crayfish is shorter than the lateral). A statocyst is present in the basal article of each first antenna (Fig 19-7B). 
            For several reasons, embryologic, morphological, and phylogenetic, the first antennae are not considered to be truly biramous even though they have two branches. Do you see anything about their anatomy that is at odds with the basic structure of the other biramous appendages you have studied? (Count the number of articles in the "protopod"). 
Figure 13. Antenna 1. Redrawn from Herrick (1909). Crab44L.gif
Figure 13
Respiratory System
            The gas exchange surfaces of decapod crustaceans consists of numerous gills, a branchial chamber to house them, and a pump to generate the respiratory current over them. The gills are associated with the proximal ends of most thoracopods.  Homarus has 20 pairs of gills (Nephrops has 19 and in crayfishes the number varies). 
            The pale, feathery gills are housed in a branchial chamber between the lateral carapace and the body (Fig 14, 19-3). You opened the left branchial chamber when you removed the left side of the carapace. The gills on that side are exposed to view and readily found. The right chamber should still be intact and covered by the right branchiostegite. The gill bailer of the second maxilla is the pump. It is assisted by the epipodite of the first maxilliped. 
"    Snip the end from one of the gills, place it in a small dish (6-cm culture dish) of water and examine it with the dissecting microscope.
            Lobsters and crayfish have filamentous (= trichobranchiate) gills in which the respiratory surface consists of numerous long filaments radiating from a central axis, rather like a bottlebrush (Fig 19-37C,D).   
            Look at the cut surface of the gill axis. Here you will see two blood channels, cut in cross-section, that extend the length of the gill (Fig 19-37C). One is the afferent channel that takes blood into the gill and the other is the efferent vessel that drains oxygenated blood away from the gill. Similarly, each filament is partitioned into two channels by a longitudinal septum. One channel is afferent, the other efferent. The lamellar gills of crabs have flat plates instead of filaments (Fig 19-37E,F). 
            The gills extend vertically into the branchial chamber from their attachments on or near the coxae of the thoracopods. Look closely and see that the gills of successive appendages are separated from each other by the long, membranous epipods of those appendages. The epipods form the boundaries of water channels that extend vertically from the free, unattached ventral edge of the carapace upwards to the attached dorsal edge. 
            Notice that the coxae of each pair of adjacent pereopods are shaped so that together they form a V-shaped inhalant channel that leads into one of the vertical channels in the branchial chamber (Fig 19-38B). There are five such inhalant channels.
            Dorsally the several vertical channels converge on an oblique, exhalant channel that runs anteriorly along the dorsal margin of the branchial chamber (Fig 19-38B). The floor of the anterior half of the exhalant channel is the epipod of maxilliped 1 and it separates the channel from the gills. The roof and walls of the channel are formed by the carapace and body. 
Figure 14. Cross section of the branchial chamber and gills of a generalized decapod. Shrimp20L.gif
Figure 14
            The gill bailer of maxilla 2 lies in the anterior end of the exhalant canal. Undulations of the bailer generate the water current that enters the inhalant canals, passes vertically over the gills, and then exits anteriorly, lateral to the mouthparts. This strong current is detectable for a distance of three animal lengths in front of the lobster. 
            A reverse current can be generated by the three maxillipedal endopods. Lobsters may reverse the current when they wish to draw water from in front of themselves over the chemosensory antennae. 
            All decapod gills are associated with thoracopods but differ in the exact location of their attachment. Podobranchs (podo=foot, branch=gill) arise on the lateral surface of the coxa of the thoracopod (Fig 14). Arthrobranchs (arthro=joint) arise on the thin articulating membrane between the coxa and the pleurite of the body wall. Pleurobranchs (pleuro=side) are attached to the pleurite dorsal to the limb articulation.  
            Look at pereopod 4 to see examples of all three types. It has one pleurobranch, one podobranch, and two arthrobranchs (one anterior and one posterior). Pereopods 2-4 each have one podobranch, one pleurobranch, and two arthrobranchs. Pereopod 5 has one pleurobranch. The cheliped (pereopod 1) and the third maxilliped each have one podobranch and two arthrobranchs. The second maxilliped has one small podobranch (plus an arthrobranch in crayfishes). Maxilliped 1 has no gills but has the important epipod that encloses the exhalant water channel and the gill bailer. There is a total of 20 pairs of gills in Homarus. (The number varies in crayfishes where the pleurobranchs are usually absent. Some have a pleurobranch on pereopod 5 and some do not.)
       Internal Anatomy    
            During this dissection be sure to keep the tissues moist as you work. It is best to conduct the dissection under water but that is difficult to do with an animal as large as a lobster. Open the body cavity carefully so you do not damage the organs within. Many of these organs are soft and lack substantial connective tissue support. Both the cephalothorax and abdomen should be opened. 
"    Open the cephalothorax as follows. Start on the left side where you have already removed the unattached carapace. Using a blunt probe, supplemented by strong scissors where necessary, free the exoskeleton of the carapace from the underlying body wall. You are not to cut through the thin body wall, rather are to separate the carapace from it. Do this all along the cut edge of the carapace on the left side. Use scissors to free the posterior margins of the carapace. 
            Insert the tip of your strong scissors under the anteroventral corner of the carapace on the left side and cut dorsally through the exoskeleton (but nothing else) toward the right side. Extend this cut all the way over the carapace and end at the anteroventral corner of the carapace on the right side. The cut will describe a transverse arch over the carapace just posterior to the base of the rostrum and the orbits. 
            Lift the cut edge of the carapace and use your blunt probe again to free the body wall from the inner surface of the exoskeleton. Free the entire inner surface and lift the carapace off the cephalothorax. Note the transverse ridge and two small apodemes near the midline of the inside surface of the carapace under the cervical groove. Set the carapace aside. 
            The body cavity (hemocoel) is not yet exposed and for it is still covered by the thin pigmented body wall. Leave the body wall intact for the time being.
            Now remove the dorsal exoskeleton from the abdomen. With scissors cut the tough connective tissue transversely along the anterior edge of the tergite of abdominal segment 1. Then insert the sharp tip of the scissors under the anterior edge of the tergite at the base of the epimeron (side plate) on the left side. Make a longitudinal cut posteriorly through the exoskeleton of the first tergite. Do the same on the other (right) side. Now lift the anterior edge of the first tergite and free it from the underlying body muscles and body wall using the blunt probe. Leave it attached to the second tergite. 
            Make cuts through the second tergite and free it from the underlying tissues as you did the first. Proceed to tergite three, then 4, 5, 6, and the telson. Remove and discard the chain of tergites. When you finish, the dorsal surface of the abdomen will be exposed. The epimera will remain in place on either side but the middle of all the tergites will be gone and the underlying muscle exposed.
            The body wall of the cephalothorax is, once the exoskeleton has been removed, almost entirely epidermis. Lift the epidermis with fine forceps and cut a hole in it with scissors. Tug on the edges of the hole to pull the epidermis away from the underlying tissue. Use a scraping (not cutting) motion of the scalpel to accomplish this separation. All you are removing is the very thin, pigmented epidermis. Nothing else! In many places you will have to separate muscles from the epidermis. These muscles insert on the inner surface of the exoskeleton but do so through the mediation of microtubules in the epidermal cells. The muscles attach to the epidermis and the epidermis to the exoskeleton.
            >1b. Make a wetmount of a small piece of the epidermis and examine it with the compound microscope. Find the irregular, stellate (starlike) chromatophores (Fig 19-44A,B). What colors of chromatophores do you find? <
            The space disclosed by the above procedure is the hemocoel. The body cavity of arthropods is part of the hemal system of the connective tissue compartment. It is not a coelomic space and is not lined by peritoneum. The visceral organs which you will now study lie in the hemocoel and are bathed in blood (= hemolymph). 
            A brief preview of the major structures in the body cavity will help you find your way around the hemocoel (Fig 15, 19-2B). The nearly shapeless, very soft, yellow-green digestive cecum fills most of the space in the cavity. The digestive cecum is called "tomale" by lobstermen. The stomach occupies most of the anterior end of the cephalothorax. It is a large, thin-walled, translucent sac. The heart is a narrow, triangular, white organ lying dorsally on the midline in the posterior cephalothorax. The heart is easily dislodged and lost during removal of the carapace. Look for it on the bottom of the dissecting pan if you cannot find it in the hemocoel. Gonads may be obvious or inconspicuous depending on age and season. Mature ovaries are dark green and testes are white. Immature ovaries are various shades of pink or red. The ovary is known as "coral" to lobstermen.
Hemal System
            Much of the hemal system is exposed by the removal of the dorsal body wall. Since it is delicate, it is best to study it first before it is destroyed. The system consists of a heart, arteries, and the open blood sinuses of the hemocoel. The blood contains hemocyanin.
"    Remove the shallow arch of muscle, connective tissue, and apodeme running transversely across the posterior dorsal margins of the cephalothorax, if it is still present.
            The heart is a large, white, triangular organ lying dorsally in the posterior half of the cephalothorax (The heart of crayfishes is an irregular rectangle). It is dorsal to the coxae of pereopods 3 and 4 (Fig 15). The sharp point of the triangle points anteriorly. Large white muscle masses lie beside, behind, and below it. Three pairs of elastic alary ligaments and muscles run from the walls of the heart to the surrounding tissues. These are stretched during systole. Their contraction, due to elastic recoil, restores the heart to its original shape during diastole.
Figure 15. Sagittal section of a lobster. Adapted from Herrick (1909).  Crab45La.gif
Figure 15
            The heart is located in the pericardial sinus, which is a region of the hemocoel (Fig 19-3A). There is no pericardium and the heart is immersed in the blood it pumps. Blood from the hemocoel enters the heart via ostia which will be discussed later. 
            Five arteries leave the anterior heart. Four of them are paired and one is unpaired. The large, unpaired ophthalmic artery (= anterior aorta) (Fig 15) exits the acute anterior end of the heart and extends anteriorly on the dorsal midline, dorsal to everything else in the body cavity. It is a slender, transparent, colorless tube running to the head. 
            Near the anterior end of the cephalothorax the ophthalmic artery expands to form an accessory heart, the cor frontale. This can be seen on the dorsal surface of the membranous dorsal wall of the stomach. It will probably be at about the level of the cut edge of the carapace. The ophthalmic artery supplies the brain and eyestalks with blood.
            A pair of antennal arteries (= anterolateral arteries) exits anterolaterally from the anterior end of the heart or from the base of the ophthalmic artery. Each extends diagonally anterolaterally across the surface of the digestive cecum and eventually ends up laterally in the head. They send branches to the stomach and stomach muscles, branchial chamber, nephridia, antennae, eyestalks, and other structures. 
            The two hepatic arteries also exit the anterior end of the heart and extend anterolaterally from it. They leave the heart posterior to the point of exit of the antennal arteries and are farther ventral. It will be necessary to push the digestive cecum, which they supply, aside to see them.   
            Two unpaired arteries exit the posterior end of the heart. There are no paired posterior arteries. One of the unpaired arteries, the dorsal abdominal artery (= posterior aorta) (Fig 15) leaves from a posteroventral protuberance of the heart and extends posteriorly along the dorsal midline into the abdominal musculature. There is a swelling at its base. It gives off lateral segmental arteries to the gonads, pleopods, posterior digestive ceca, and muscles of the abdomen. 
"  With scissors make a middorsal incision along the length of the abdomen. Do this under magnification and cut only deep enough to expose the dorsal abdominal artery and its segmental branches. Do not cut or damage any tissue in the abdomen except the muscles and body wall. The intestine lies immediately ventral to the artery. You will also see the digestive ceca and gonad in the space between the muscle masses in the anterior abdomen.
            The sternal artery is the second of the two unpaired posterior arteries (Fig 15). Find the origin of the sternal artery but do not attempt to trace it now. It runs ventrally from the posterior end of the heart (Fig 15,19-3A). In doing so it passes to either the right or left of the intestine and then penetrates the nerve cord by passing between the right and left connectives between the ganglia of thoracomeres 7 and 8. Upon arrival at the ventral part of the body dorsal to the sternites it bifurcates into a ventral thoracic artery to thoracopods 1-6, the mouthparts and anterior nerve cord and a ventral abdominal artery to thoracopods 7 and 8 and the posterior nerve cord (Fig 15, 19-2B). 
            Three pairs of ostia penetrate the walls of the heart (Fig 15, 19-2B). Ostia are pores through the wall of the heart through which blood enters the heart during diastole. Each is equipped with a no-return valve that prevents escape of blood during systole. The heart of Homarus has dorsal, lateral, and ventral pairs of ostia. The dorsal pair is not immediately apparent in dorsal view because it is overhung by a thin shelf of tissue.
"    Remove the heart and place it in a 6-cm dish of water and look for its six ostia with the dissecting microscope. Lateral and ventral pairs are easily found but the dorsal pair is hidden by a small sheet of tissue.
Reproductive System
            The reproductive system consists of a pair of gonads which together form a single H-shaped organ connected to the exterior by a pair of gonoducts. (The gonad is Y-shaped in crayfish with the stem of the Y pointed posteriorly and the two arms anteriorly.) The size of the gonad depends on the season and state of maturity of the specimen. They may be very small, very large, or anything in between. The gonad occupies a coelomic remnant, one of the few remaining in arthropods, and the gonoducts are ancient coelomoducts. The gonad lies on the floor of the pericardial sinus immediately ventral to the heart (Fig 15, 19-2B). Its arms may extend far anteriorly and posteriorly.
"    Remove the heart if you have not already done so. Remove as much digestive cecum as necessary to expose the gonad. If you are dissecting out of water it will help to rinse the body cavity with a gentle stream of water occasionally.
            The ovary (Fig 15) has the shape of a very long H with its vertical arms oriented longitudinally and the crossbar transversely. The crossbar of the H is on the floor of the pericardial sinus at the approximate level of pereopod 3. The two longitudinal lobes lie laterally and may extend from far back in the abdomen to the anterior end of the stomach. In juveniles they are white but as they mature they successively become yellow, salmon, pale green and finally dark green. After oviposition the ovary is temporarily gray. Eggs can be seen within the transparent walls of mature and maturing ovaries. A straight oviduct exits each lateral arm just a little posterior to the crossbar and extends ventrally to the female gonopore on the coxae of the third pereopods (thoracomere 6). 
            The testis (Fig 19-2B) is usually H-shaped (Y-shaped in crayfish) although the crossbar may sometimes be absent. It occupies the same position on the floor of the pericardial sinus as the ovary. A coiled vas deferens (Fig 19-2B) exits each side of the testis and extends to a male gonopore on the coxa of pereopod 5 (thoracomere 8). The testis is white and may be confused with the digestive ceca. 
            During copulation the male and female oppose their ventral surfaces (face each other) with the male holding the female in position (Fig 19-47A). The male gonopods (pleopod 1) are held together and inserted into the seminal receptacle of the female. The genital papilla of the male delivers sperm to the base of the gonopods. The sperm travel through grooves in the gonopods to reach the seminal receptacle. 
            The sperm remain up to two years in the receptacle. When the female deposits eggs, the sperm leave the receptacle and fertilize them. The eggs are covered by a gluelike secretion that sticks them to the setae of the female pleopods. The eggs are brooded on the pleopods until they hatch into planktonic larvae. (The eggs of crayfish hatch into miniature crayfish which remain with the mother for a time.)
Digestive System
            The digestive system consists of an ectodermally lined foregut, endodermal midgut, and ectodermal hindgut. The ectodermal portions are lined by epidermis which secretes an exoskeleton, or cuticle that is molted with the rest of the exoskeleton. 
            The mouth is located on the ventral surface of the head between the two mandibles (Fig 15). 
      Insert a blunt probe into the space between the cutting edges of the mandible and slip it gently into the mouth. Push it vertically (gently) until you can see it pushing into the roof of the stomach.
            The stomach (= proventriculus) is located directly over the mouth, as you have just demonstrated, and the short esophagus connecting the two is a vertical tube (Fig 15, 19-2B). You will see the esophagus later. The stomach and esophagus make up the foregut. The stomach consists of two chambers. Anteriorly is the large, wide, hard cardiac stomach and posteriorly is the smaller, narrower, softer pyloric stomach(Fig 16, 19-34). The intestine, or midgut, exits the posterior ventral end of the pyloric stomach.
            The two digestive ceca (right and left), also known as the digestive glands, liver or hepatopancreas, are large, soft, lobulated organs filling most of the space beside the stomach and heart (Fig 15). Their true shape can be seen only when immersed in water. Each connects by a duct to the anteriormost end of the midgut in the area between the pyloric stomach and the intestine (Fig 16). Try to find this connection. 
"  After you have studied the digestive ceca, remove them. This is easiest to do by immersing the animal in water or by running a gentle stream of water over the body cavity to support the ceca which can then be removed in pieces with forceps.
            With the digestive ceca out of the way many features of the body cavity are easier to see. You may want to take another look at the gonads and gonoducts. If your specimen is reproductive and has very large gonads you should remove them at this time also.
            Look at the stomach again. The cardiac stomach contains the gastric mill which is a series of calcareous plates and teeth that grind, mix, and regrind the food (Fig 16). Preliminary trituration of the food takes place at the third maxillipeds and mandibles but the chief trituration occurs in the cardiac stomach. Ungrindable or indigestible materials are regurgitated. 
            The gastric mill grinds the food exceedingly fine and it is then filtered by a sieve, the filter press (Fig 16, 19-34), of setae in the pyloric stomach. Solutes and ultrafine particles then enter the digestive ceca. Large particles must be either reground or regurgitated. Most hydrolysis and absorption takes place in the digestive ceca. 
"    Open the cardiac and pyloric stomachs with a median, dorsal longitudinal incision.
      The complex structure of the gastric mill of the cardiac stomach and the sorting system of the pyloric stomach will be immediately apparent (Fig 16). The stomach has over 30 hard exoskeletal elements and 14 muscles to operate them. The ossicles of the gastric mill are part of the exoskeleton and are lost and replaced with each molt. The cuticular lining breaks into pieces during ecdysis and the pieces are voided through the intestine. Find some of the larger ossicles in the cardiac stomach. Some of them look very much like mammalian molars (Fig 16). Find the setal filter press in the pyloric stomach. 
The gastroliths are two large, oval calcified areas, one on each anterolateral wall of the cardiac stomach (Fig 16). Prior to each molt calcium carbonate is secreted here to form large, hard, white pads. They are areas of calcium storage protected by the cuticular lining of the stomach from dissolution by stomach acid. The loss of the cuticle during the molt removes this protection and the calcium dissolves in stomach fluid. It is then absorbed into the blood and becomes available for deposition in the newly formed exoskeleton. 
            Fin the short esophagus connecting the cardiac chamber with the mouth (Fig 15, 16, 19-2B).
            The anterior end of the midgut, or intestine, is swollen and resembles the pyloric stomach which it exits (Figs 15, 16). The ducts of the digestive ceca open into this swollen region. 
Figure 16. Sagittal section of the stomach of the lobster, Homarus. After Herrick (1909). Crab46La.gif
Figure 16
            The intestine extends posteriorly as a straight tube and enters the abdomen where it passes through a median space between muscle masses. It can be seen ventral to the dorsal abdominal artery by pushing the muscle masses apart. The intestine extends posteriorly to abdominal segment 6 where it ends at a sphincter
            Beyond the sphincter the gut is wider and is the rectum (Fig 15). The rectum is the hindgut. Immediately anterior to the sphincter the large, simple, tubular posterior midgut cecum arises from the dorsal wall of the intestine and extends posteriorly to the end of abdominal segment 6 (Fig 15). It lies atop the rectum and is about equal to it in diameter,
            The rectum continues on to open at the anus on the ventral surface of the base of the telson. (In crayfish the situation of the mid- and hindguts is different. The midgut is very, very short. It receives the two ducts of the digestive ceca, gives rise to an anterior midgut cecum that arches over the pyloric stomach and lies on its surface. Posterior to the point of exit of the anterior midgut cecum the gut is rectum (hindgut.)
Excretory System
"    Cut the esophagus and intestine and remove the stomach.
            The excretory organs of decapods are a pair of highly modified metanephridia located in the segment of the second antenna. They are saccate nephridia variously known as kidneys, nephridia, green glands, antennal glands, or coxal glands. Each consists of a glandular region which contains a remnant of the coelomic space called the end sac, a convoluted tubule, and a thin-walled bladder that empties to the exterior via the nephridiopore on the coxa of antenna 2 (Fig 19-6B).
            The antennal gland is a large pale greenish organ closely adhering to the anterolateral wall of the head immediately posterior to the base of the second antenna (Fig 15). The bladder is a very large, thin-walled, transparent sac lying atop the gland. It is not readily apparent that it is a sac and usually looks like a simple transparent membrane.
Nervous System
            The brain, or supraesophageal ganglion, is a white mass located on the midline of the anterior wall of the head between the bases of the two eyestalks (Figs 15, 16, 19-2B). Four major pairs of sensory nerves enter it. In addition, a pair of large circumesophageal connectives leave it posteriorly. Three pairs of the sensory nerves are easily seen but the fourth exits the ventral surface of the brain and requires some additional dissection to reveal it. 
            Carefully remove any connective tissue remaining over the surface of the brain and find the three dorsal pairs of nerves. The anteriormost is the short optic nervethat run posteriorly from the eyestalks (Fig 16-10A). The middle pair consists of the longer tegmental nerves from the epidermal sense organs of the dorsal head. The third and posteriormost pair is the antennal nerve from antenna 2. The fourth pair, entering the ventral surface of the brain, comes from the chemoreceptors of the first antenna, or antennula. These are the antennulary nerves. Each exits the base of a first antenna and enters the ventral surface of the posterior brain, almost as far posterior as the antennal nerves. 
            Two large circumesophageal connectives leave the posterior border of the brain and pass posteriorly, one on either side of the esophagus (Fig 16). Posterior to the esophagus they join the median subesophageal ganglion, which is not yet visible. A small tritocerebral commissure extends transversely from one connective to the other immediately posterior to the esophagus. Its functional significance is slight and it is of more interest phylogenetically. Its presence is evidence that the currently preoral tritocerebrum was once postoral.
            Posterior to the tritocerebral commissure, the thoracic portion of the nerve cord, including the subesophageal ganglion, lies deep below the internal skeletal supports of the thorax. It is difficult to uncover the subesophageal ganglion and nerve cord and the remainder of the nervous system dissection may be omitted if time is short.
"     To reveal the ventral portions of the nervous system use strong scissors to cut posteriorly through the skeletal apodemes (= endophragmal shelf) in the floor of the cephalothorax in order to trace the circumesophageal connectives posteriorly beyond the esophagus. Upon reaching the abdomen, use a scalpel to cut the transverse connections between right and left ventral muscle masses and remove those longitudinal columns of muscle. The nerve cord will then be seen lying on the inner surface of the abdominal sternites.
            The subesophageal ganglion (Fig 16) innervates the mandibles, maxillae, and maxillipeds and is formed of the fused segmental ganglia of their segments. The double ventral nerve cord proceeds posteriorly from the subesophageal ganglion and bears paired segmental ganglia in the floor of each segment beginning with thoracomere 4, which is the segment of the chelipeds (Fig 15). The double nerve cord runs posteriorly to the sixth abdominal segment with a segmental ganglion in each segment.
            Bullough WS . 1958. Practical Invertebrate Anatomy 2 nd ed. MacMillan, New York. 483p.
            Govind CK. 1989. Asymmetry in lobster claws. Am. Sci. 77:468-474.
            Herrick FH. 1909. Natural History of the American Lobster. Bull. Bur. Fish. 26:150-408, pls 33-47.
            Huxley TH. 1880. TheCrayfish, An Introduction to the Study of Zoology. Appleton, New York. 371p. (Reprinted 1973, M.I.T. Press, Cambridge.)
            Lochhead JH. 1950. Crayfishes (and Homarus) in F. A. Brown (ed) Selected Invertebrate Types. Wiley, New York. Pp 422-447.
Ruppert EE, Fox RS, Barnes RB.  2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp. 
            Snodgrass RE . 1952. A Textbook of Arthropod Anatomy. Cornell Univ. Press, Ithaca. 363 p. (reprinted 1971 by Hafer Publishing, New York) (crayfish on pp 142-179).
Dissecting microscope
Compound microscope
Lobster or crayfish, living or preserved
Large dissecting pans
#1 insect pins
8-cm culture dishes
Isotonic magnesium chloride for living specimens
Dissecting set