Invertebrate Anatomy
OnLine
Homarus americanus ©
American Lobster
with notes on crayfish
20jun2006
Copyright 2001 by
Richard Fox
Lander University
Preface
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.
Systematics
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
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
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.
Thoracopoda
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.
Phyllopodomorpha
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.
Tagmata
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
Carapace
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.
Thorax
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.
Abdomen
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.
Appendages
General
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 3. Pleopod 3. Redrawn from Herrick (1909).
Crab34L.gif
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
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.
Pereopods
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
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).
Chelipeds
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? <
Gonopores
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).
Maxillipeds
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
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
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 9. Maxilla 2. After Herrick (1909). Crab40La.gif
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 11. Mandible. Redrawn from Herrick (1909).
Crab42L.gif
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
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
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
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).
Preview
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
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.
Female
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).
Male
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.
Copulation
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
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.
References
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).
Supplies
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