Invertebrate Anatomy OnLine
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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, Astacoidea SF, Cambaridae F, (Fig 16-15, 19-67, 19-90)
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 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 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.
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).
Astacidea iO includes the clawed lobsters and the freshwater crayfishes. This exercise can be used with any crayfish or clawed lobster. Preserved crayfishes are provided by biological supply companies without indication of identity but most North American species belong to Procambarus (115 taxa), Cambarus (82 taxa), or Orconectes (76 taxa). Pacifastacus has eight species, restricted to Pacific coast watersheds and Missouri River headwaters. The study should be conducted on the stage of a dissecting microscope. Living specimens should be anesthetized with chloroform-saturated water.
Place a living or preserved crayfish in a dissecting pan of appropriate size and take it to your bench for study. Crayfish have bodies similar to that of the presumed ancestral crustacean. Such a body is essentially shrimplike in that it is elongate and nearly cylindrical in cross section. The abdomen is well developed, its segmentation is readily apparent, and appendages are present on every segment. Note the bilateral symmetry of the animal and find anterior, posterior, dorsal, ventral, right, and left.
The body is covered by a hard, jointed, nonliving cuticle, or exoskeleton secreted by the underlying epidermis. In general, the body wall consists of little more than the exoskeleton and the epidermis beneath it because an animal with a rigid exoskeleton has little need for any additional body wall. The original body wall muscles have become specialized individual muscles and no longer form continuous layers in the body wall as they do in the wormlike ancestors of the arthropods. Continuous layers of circular and longitudinal muscles would be inefficient under a solid and immovable exoskeleton. The exoskeleton provides strength, structural support, and protection so connective tissue is not needed. The body cavity is a hemocoel, not a coelom, so there is no peritoneum. Only the epidermis is essential and must be retained because it secretes the exoskeleton.
The exoskeleton 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 surfaces of the exoskeleton and moves its many parts. The exoskeleton between adjacent regions is thin and flexible to permit motion. A complicated "endoskeleton" composed of internal processes, called apodemes, extends internally from the inner surface of the exoskeleton. The gills and the anterior and posterior regions of the gut are covered with epidermis and a thin exoskeleton.
Many parts of the exoskeleton bear small, articulated, movable bristles called setae. These can be seen over most of the body. Thicker articulated processes from the exoskeleton are spines. Simple outgrowths of the exoskeleton that are not articulated are usually called teeth.
The body is composed of a linear series of segments, or somites. The malacostracan body consists of 19 segments but you cannot see or count them all. Each one, however, bears a pair of jointed appendages, which are visible and countable. In the ancestral crustacean all segments were identical, or nearly so (homonomous), as were their appendages. In derived crustaceans the segments and their appendages are specialized for various purposes and for the most part no longer resemble each other closely (heteronomous).
Groups of adjacent segments and their appendages tend to have similar functions and together accomplish certain specialized tasks. This results in a regionalization of the body into tagmata. In crustaceans there were originally three tagmata; the head, thorax, and abdomen. The crustacean head is always composed of five segments but the thorax and abdomen are variable. Within Malacostraca there is no variability in segment number and there are always eight segments in the thorax and (almost) always six in the abdomen.
It is common in crustaceans for the head to fuse with some anterior thoracic segments to form a new tagma, the cephalothorax. The appendages of these thoracic segments are modified to serve as mouthparts and are known as maxillipeds. The remaining unaffected thoracic segments form another new tagma, the pereon. Such secondary tagmosis occurs in Decapoda where the cephalothorax consists of the five head segments and the first three thoracic segments and bears five pairs of head appendages and three pairs of maxillipeds. The remaining five thoracic segments are the pereon and their appendages are pereopods. Most authors use different rules for decapods and refer to the head and entire thorax as the cephalothorax even though only three appendages are fused with the head. That custom is not followed here.
In Malacostraca the cephalothorax and most of the pereon is covered dorsally and laterally, but not ventrally, by a double sheet of exoskeleton called the carapace (Fig 1, 19-19). Although you cannot tell it by looking at the animal, the carapace is an outgrowth of the exoskeleton of the last 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 (Fig 8). The outer wall of the carapace is sclerotized and covered by a thick exoskeleton and is hard and strong but the inner wall has only a thin exoskeleton and is transparent and flexible.
The segments of the cephalothorax are fused together and cannot be told apart but those of the pereon remain independent and are not fused together, even though dorsally and laterally they appear to be. The carapace that covers them is not segmented but it, remember, is not part of the pereon. Under the carapace, the pereon is segmented. The segmentation of the thorax is apparent ventrally where it is not covered by carapace. No such segmentation can be seen in any view of the cephalothorax. A conspicuous transverse dorsal groove divides the carapace into an anterior 1/3 and posterior 2/3. This is the cervical groove and it marks the boundary between head and thorax.
Figure 1. A dorsal view of an American lobster, Homarus americanus. Adapted from Herrick, 1909.
Anteriorly the carapace bears a conspicuous anterior, median, pointed process called 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 an eyestalk with a compound eye at its distal end. The black, multifaceted cornea of the eye covers almost of the entire circumference of the stalk.
The thorax is composed of eight segments, called thoracomeres, and as we have seen, all eight are hidden beneath the carapace. Each thoracic segment bears a pair of appendages, or thoracopods. The anterior thorax, consisting of three segments, is fused with the head to form the cephalothorax. 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 pereomeres. The pereon is not part of the cephalothorax even though it is covered by the carapace. Do not confuse the carapace with the cephalothorax. They are not the same thing. The carapace is a fold of the body wall which, in decapods, covers the cephalothorax and pereon.
The abdomen of primitive decapods is well developed with clearly visible segments and powerful longitudinal muscles. It is this abdominal musculature that is primarily responsible for the culinary popularity of shrimp, lobsters, and crayfish. The abdomen is also known as the pleon and its segments are pleomeres. Count the abdominal segments. There are six of them, all are clearly visible, with none fused with another or with the thorax. The posterior end of the body is not a segment. It is the telson. If you counted it as a segment, you came up with the wrong number. The anus is located on the ventral side of the telson.
The exoskeleton of the abdominal segments of the crayfish approximates the typical ancestral condition. Primitively, each body segment is enclosed in four articulated exoskeletal plates, or sclerites, that form a complete ring around the segment. Dorsally is the tergite, ventrally the sternite, and laterally there are two pleurites, one on each side.
In astacideans (lobsters and crayfish) the tergite and pleurites are fused together to form a hard arch of exoskeleton covering the dorsal and lateral aspects of the segment. On segments 2-6 the pleurites extend ventrally past the body as side plates, or epimera, which together form a shallow ventral channel below the body of the abdomen.
The sternites cover most of the ventral surface of the abdomen but the pleurites cover its lateral parts. For the most part sternites are thinner and more flexible than tergites and pleurites. They are transparent and, in living specimens, the abdominal musculature and nerve cord can be seen through them. The posterior margin of each sternite, however, is 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.
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 numbered in the opposite direction, from anterior to posterior.
Before beginning your study of crayfish appendages it might be a good idea to review the morphology of a typical crustacean appendage. Arthropod appendages are paired, with one pair per segment. Each appendage is a linear series of articles joined by flexible articulations. Appendages may be biramous with two branches, or uniramous, with only one branch.
A biramous appendage has a basal article attached by its proximal end to the body. From its distal end arise two rami, or branches (Fig 2, 19-3B, 19-4A). The basal article is the protopod. Often the protopod is divided into two articles, the coxa and basis. The two rami are an outer, or lateral, exopod and an inner, or medial, endopod. The two rami may be composed of any number of articles depending on what they are specialized to accomplish. They may be similar to each other or different. If only one ramus is present and the appendage is said to be uniramous (Fig 3). Sometimes additional branches of the protopod or rami are present. Any additional branch on the lateral side of the appendage 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.
Study the abdominal appendages of your specimen but do not remove them. Each of the six abdominal segments bears a pair of appendages. Most of these are biramous. The last (posteriormost) pair of abdominal appendages, located on abdominal segment 6, are uropods (Fig 3, 19.2B).
Figure 2. A lobster (Homarus americanus) uropod. Redrawn from Herrick (1909).
The uropods have a relatively small protopod and two large, flat rami. The exopod is biarticulate (has 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 telson is neither an appendage nor a segment). The four rami plus the telson make up the tail fan, which functions as a large paddle. With the fan deployed, flexure of the powerful abdominal muscles moves the fan rapidly forward under the body and results in the generation of a forward jet of water that propels the animal backwards.
The remaining five pairs of abdominal appendages are pleopods 1-5 (counting from anterior to posterior). Pairs 2-5 are biramous and are similar to each other. They are narrow and whiplike, although not very long (Fig 19-12B). The pleopods of females are better developed than those of males and are used to carry the eggs, which are glued to the fringe of setae around the rami.
The first pleopods of males are uniramous and are modified to serve as intromittent organs to transfer spermatozoa to the female. In males the first pleopods are uniramous and are referred to as gonopods. Adult male crayfish can be either first form or second form. The gonopods of first form males are sclerotized and hard and suitable for intromission. Those of second form males are unsclerotized and soft and cannot transfer sperm to the female. The first form gonopods have a species specific shape that fits like a key into a seminal receptacle, the annulus ventralis, with a corresponding shape, like a lock, on the female venter. If you have a male determine if it is first form or second form.
Using your own specimen if you have a first form male or a borrowed one from another student, identify the specimen to genus using the sculpturing of the 1 st pleopod. In the genus Procambarus the male first pleopod usually terminates in more than two processes. In the genus Cambarus, the first form male first pleopod has two or less terminal processes and they are bent at right angles to the shaft of the pleopod. In the genus Orconectes the first form male first pleopod has two or fewer processes and they are not bent. They arise at the end of the shaft or from its posterior side and are parallel to the shaft or slightly curved.
Pereon and Pereopods
Each thoracic segment bears a pair of appendages but just as there are two distinctly different regions of the thorax, there are two distinctly different types of thoracic appendages. The appendages of the anterior three thoracic segments are maxillipeds, are part of the cephalothorax, and function as auxiliary mouthparts. The appendages of the posterior five thoracic segments (pereomeres) are pereopods and function as walking legs or pincers. They are part of the pereon.
The five segments of the pereon bear a total of 10 appendages which accounts for the name Decapoda (= 10 feet). All decapods have five free thoracic segments and five pairs of pereopods. The ten appendages are usually referred to loosely as "walking legs" whether or not they are used for walking. All pereopods lack the exopod and thus are uniramous. The endopod is long and narrow (Fig 3). This shape of ramus is referred to as stenopodous in contrast to a broad, flat, leaflike phyllopod such as the uropod.
" Lift the ventral edge of the carapace and note that it is attached dorsally to the thorax but is free laterally. With strong scissors cut away the unattached lateral portion of 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. This space is the branchial chamber and it contains the gills. The gills, which are outgrowths (epipodites) of the thoracic appendages, will be studied later. They are feathery, white, filamentous processes. Keep them moist so they do not dry out. Removal of the carapace exposes the entire length of the pereopods and makes it easier to study them. The lateral wings of the carapace are the branchiostegites (Fig 8). The branchiostegites enclose the branchial chamber. You just removed the left branchiostegite.
Look at the middle walking leg. It is pereopod 3 (Fig 3, 19-20). The typical malacostracan thoracopod is composed of seven articles. The two proximal articles are the subdivided protopod and the distal five are the five articles of the endopod. The seven articles are, in order from proximal to distal; coxa, basis, ischium, merus, carpus, propodus, and dactyl. Find the seven articles of pereopod 5. The proximal article is the coxa. It is wide and short and articulates with the sternite of the third pereomere. Distally it articulates with a short, narrow basis. 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 crayfish can deliberately autotomize (auto = self, tome = cut) its limb. This plane is specialized for this function and the animal can loose its limb, at this plane only, without trauma or significant blood loss. It usually replaces the limb with subsequent molts.
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.
Figure 3. Pereopod 3 of the lobster, Homarus americanus. Adapted from Herrick, 1909.
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. 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 one is said to be 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.
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. The gills will be considered later.
Pereopods 4 and 5 are almost identical to each other and differ from 1-3 in being simple, rather than chelate. The coxa of pereopod 4 has a large membranous, leaflike epipod that is absent from 5. This epipod extends vertically between the gills. Similar epipods are present on the pereopods 1-3 and maxilliped 3. The first pereopods are much larger than any other appendage. 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 and the chelae, as expected, are formed of the propodus and dactyl.
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 joints of the cheliped.
The female gonopores are the external openings of the oviducts. They are located on the medial side of the coxa of pereopod 3 (= thoracomere 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 (= penes) on the medial surface of the coxa of pereopod 5 (= thoracomere 8). The position of the male and female gonopores on these segments is constant throughout Malacostraca.
A conspicuous oval annulus ventralis (= seminal receptacle) is located on the ventral surface of the female pereon between the coxae of pereopods 4 and 5. It bears a deep median cleft, known as achink. The male gonopod is inserted into the chink where it deposits sperm into the recess. If your specimen is a male, find a female crayfish and look at the annulus.
In the genus Procambarus the annulus ventralis is surrounded by flexible cuticle and is freely movable (but may be partly hidden by an overhang of the preceding sternite). The annulus ventralis of Orconectes is inflexibly fused to the sternum immediately anterior to it. In Cambarus the annulus ventralis is not uniformly sclerotized and, even though fused with the sternite, is capable of slight hingelike motion between the anterior and more heavily sclerotized posterior portion. Study a female crayfish and see if you can identify it to genus using these features of the annulus ventralis.
During copulation the male turns the female over so their ventral surfaces face each other. The male uses his chelipeds to hold the female in position. The male gonopods (pleopod 1) are held together and inserted into the annulus ventralis of the female. The male genital papillae deliver sperm to the base of the gonopods. The sperm travel along grooves in the gonopods to the female annulus where they are stored, sometimes for weeks or months before being used for fertilization. Immediately before shedding eggs the female secretes a glue-like glair onto the ventral surface of the abdomen and its pleopods. She then releases sperm from the annulus onto the glair. Next she releases eggs from her gonopores on thoracomere 6 onto the glair-covered pleopods. The eggs are fertilized and stick to the pleopods where they are then brooded until they hatch into miniature crayfish which remain associated with the mother for a time.
The anterior three pairs of thoracic appendages are maxillipeds. Unlike the pereopods, the maxillipeds are biramous. The third maxillipeds are on the third thoracomere and are immediately anterior to the chelipeds. Each is large and intermediate in shape and size between the heavy, robust legs of the pereon and the delicate mouthparts of the head. Each has a large, stenopodous endopod and a small filamentous exopod (Fig 4, Fig 19-2A). The protopod is divided into a coxa and a basis, as it is in all thoracic appendages. The small exopod 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 4. Maxilliped 3 of the lobster Homarus. Redrawn from Herrick (1909).
Hold the third maxillipeds aside and look at the next appendage. It is the second maxilliped. It too is biramous but is much smaller that the third. Its exopod is longer than its endopod.
The first maxilliped is the appendage of the first thoracomere. Its exopod resembles those of the other maxillipeds and is long and narrow. Its endopod is short and inconspicuous. There are two large wide, thin endites that cup over the bulge of the mandible. The long posterior epipod extends posteriorly into the branchial chamber.
The remaining five pairs of appendages are those of the five head segments. The posterior three are mouthparts whereas the anterior two are antennae with a sensory function.
The second maxilla is the appendage of the fifth and posteriormost head segment and it lies immediately anterior to the first maxilliped. It generates the water current that pumps water out of the front of the branchial chamber. Its basal portion bears four flat, narrow endites, a slender endopod, and a long flat gill bailer, whose movements generate the respiratory current through the branchial chamber. The gill bailer, also known as the scaphognathite, is composed of the exopod and epipod (Fig 5). The 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. It functions in concert with the gill bailer of the second maxilla. The bailer lies in and beats in the trough formed by the epipod of the first maxilliped.
Figure 5. Maxilla 2 of the lobster, Homarus. After Herrick (1909).
The first maxillae are small and more delicate than the second. They are the smallest of the mouthparts and lie curved tightly against the smooth, hard surface of the mandible. Each has two broad endites and a narrow, larger, endopod. There is no exopod.
The mandibles are the most anterior of the mouthparts. They are heavily calcified and equipped with powerful muscles. There is a large basal portion which bears a cutting edge on a medial lobe. A three articled palp arches 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 both sensory antennae (Fig 1, 19-2A). The biramous second antennae are by far the larger of the two pairs (Fig 7, 19-2B). Each arises by a biarticulate protopod consisting of a proximal coxa and a distal basis. The short, wide exopod is called the antennal scale. It 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. The lower surface of the coxa bears a small circular tubercle with an opening in its center. This is the nephridiopore, the external opening of the kidney.
Figure 6. Mandible of the lobster, Homarus. Redrawn from Herrick (1909).
The first antennae (= antennules), are situated below the eyestalks. They are much smaller than the second antennae. Each has a triarticulate basal stalk from which arise two slender multiarticulate flagella. There is a statocyst in the basal article of each first antenna.
The two eyestalks are also on the anterior head. Each bears a conspicuous compound eye at its distal end.
The respiratory apparatus of decapod crustaceans consists of numerous gills, a branchial chamber to house them, and a water pump to generate a respiratory current over them. The gill bailer of the second maxilla is the pump. The number of gills in crayfishes is 17-18 pairs and lobsters have 20 pairs. The gills are epipods attached to the coxa or the adjacent articulating membrane or pleurite of most thoracopods.
The pale, feathery gills are housed in the branchial chamber between the lateral carapace and the body (Fig 8, 19-3A). You opened the left branchial chamber when you removed the left carapace and the gills on that side are exposed to view. The right chamber should still be intact and covered by the branchiostegites of the carapace. Note that the branchial chamber is outside the body and is filled with water, even though it is under the carapace and appears to be internal.
The gills extend vertically into the branchial chamber from their attachments on or near the coxae of the thoracopods (Fig 19-38B). 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 (Fig 19-38B). Notice that the coxae of each pair of adjacent pereopods are shaped so that together they form V-shaped inhalant channels that lead into one of the vertical channels in the branchial chamber. There are five such inhalant channels.
Figure 7. Antenna 2 of the lobster, Homarus. The ventral surface and nephridiopore are shown in the inset. Redrawn from Herrick (1909).
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 this channel is made of the epipod of maxilliped 1 which separates the channel from the gills. The roof and walls are formed by the branchiostegite of the carapace and body. The gill bailer of maxilla 2 lies in the anterior end of the exhalent canal in the epipod of maxilliped 1. Undulations of the bailer generate the negative pressure that draws water in the inhalant canals, vertically over the gills, and then anteriorly, to exit lateral to the mouthparts.
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 8). 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 but are usually absent in crayfishes.
Look at pereopod 4. Like most crayfish thoracopods it has one podobranch and two arthrobranchs (one anterior and one posterior). Pereopods 1-4 and maxilliped 3 each have one podobranch and two arthrobranchs. Pereopod 5 may or may not have a pleurobranch, depending on taxon. Maxilliped 2 has one small podobranch and an arthrobranch. Maxilliped 1 has no gills but has the important epipod that encloses the exhalant water channel and the gill bailer. Some crayfishes have a pleurobranch on pereopod 5 and some do not. Crayfishes thus have a total of 17-18 pairs of gills whereas lobsters, with several pleurobranchs, have 20.
Figure 8. Cross section of the branchial chamber and gills of a generalized decapod.
" Snip the end from one of the gills, place it in a 6-cm culture dish of water and examine it with the dissecting microscope.
Crayfishes and lobsters 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 unoxygenated 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.
Watch the video "America's crayfish, Crawling in troubled waters", produced by Virginia Tech and the U.S. Fish and Wildlife Service.
Pending completion of this section, the description of the internal anatomy of Homarus americanus (American lobster) in this series ( Invertebrate Anatomy OnLine )can be substituted .
Bullough WS. 1958. Practical Invertebrate Anatomy 2 nd ed. MacMillan, New York. 483p.
Herrick FH. 1909. Natural History of the American Lobster. Bull. Bur. Fish. 26:150-408, pls 33-47.
Hobbs HH. 1991. Decapoda, in Thorp JW, Covich AP (eds). Ecology and classification of North American freshwater invertebrates. Academic Press, San Diego.
Huxley TH. 1880. The Crayfish, 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. pp422-447.
Pennak, R.W. 1989. Freshwater invertebrates of the United States, 3ed. Wiley.
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 pp 142-179).
Living or preserved crayfish
Chloroform-saturated water for living specimens.
Video projection equipment