Invertebrate Anatomy OnLine
with notes on Cancer
Copyright 2001 by
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.
Mandibulata, Crustacea sP,
Eucrustacea, Thoracopoda, Phyllopodomorpha, Ostraca, Malacostraca C,
Eumalacostraca, Caridoida, Decapoda O, Brachyura sO,
Brachyrhyncha iO, Portunoidea SF , Portunidae F (Fig
16-15, 19-67, 19-90)
by far the largest and most diverse animal taxon, includes chelicerates,
insects, myriapods, and crustaceans as well as many extinct taxa such as
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.
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,
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.
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
organs also vary with taxon and include many types of gills, book lungs, and
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.
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.
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.
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
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.
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
compound eyes are stalked primitively although derived sessile eyes occur in
includes most of the large and familiar crustaceans such as crabs, shrimps,
lobsters, crayfish, isopods, amphipods, and others. 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 gonopores are on the eighth thoracic segment, male on the sixth.
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).
blue crab, Callinectes
sapidus , is a large
crab of shallow waters along the east coast of North and South America. Callinectes belongs
to the swimming crab family, Portunidae, most of whose members have large
oarlike hind legs that are used for swimming. This
exercise was written for Callinectes but
any portunid crab, such as Portunus or Ovalipes,
will serve equally well. Parenthetical
comments refer to a similar genus, Cancer,
in the family Cancridae.
dissection can be performed on preserved, frozen, or living anesthetized
markets sometimes offer steamed crabs and even these can be used for a
laboratory exercise of external anatomy.
crabs can be anesthetized in about 20 minutes by immersion in carbonated water
or chloroform-saturated seawater. The
inhalant aperture of the gill chamber must be immersed in the liquid in order
for anesthetization to occur.
for about 40 minutes will kill the crabs and provide specimens suitable for
specimens must be handled with caution as they are quick, willing, and able to
inflict painful wounds. With
experience, they can be handled safely and confidently by firmly grasping, with
thumb and forefinger, the base of one of the two swimming legs. Don’t
try this if you don’t know what you are doing. Blue
crabs really hurt. The pincers of unanesthetized crabs should be inactivated
with small rubber rings made by cutting cross sections of surgical rubber
sapidus supports an important
fishery along the east coast of North America and is an important commercial
Crabs are notoriously fast, pugnacious, evil-tempered, and voracious. They
are omnivores that feed on a variety of living and dead animal and plant
of its economic importance, much is known of its biology. Mature
females spawn in high salinity water near the mouths of estuaries and the
planktonic larvae make their way into the estuary to lower salinity upstream
settle to the bottom and begin a benthic existence, molting, growing, and moving
downstream to higher salinities. Mating
occurs only once in the life of the female and afterwards she migrates to the
mouth of the estuary and the nearby ocean. Here
she spawns, using stored sperm to fertilize her eggs. Planktonic
larvae hatch from the eggs and begin the journey back into the estuary.
the most derived malacostracan morphology and shows several departures from that
of the ancestral crustaceans. Notable
among these are the extreme reduction of the abdomen, the cephalization of the
nervous system, and the shortening, broadening, and flattening of the body. As
you conduct the dissection, think about the morphology of a more primitive
malacostracan such as a shrimp, lobster, or crayfish and consider how it differs
from that of crabs.
Body and Tagmata
the crab and note that, unlike more primitive decapods such as shrimps and
crayfish, the body is short, very wide, and dorsoventrally
depressed (Fig 1, 19-31). The
ancestral crustacean tagmata of head, thorax, and abdomen have been reorganized
into cephalothorax, pereon, and abdomen but the abdomen is reduced and almost
malacostracan head has five segments, the thorax eight, and the abdomen six. The
head and first three thoracic segments are fused to form a new tagma, the cephalothorax leaving
five free thorax segments as the pereon. The reduced abdomen is
folded beneath the thorax and is not visible dorsally. Most
of the crab body is cephalothorax and pereon which are covered dorsally by a
large, hard, shield-like carapace. The
carapace is an outgrowth of the most posterior head segment.
decapods, the first three thoracic segments are fused with the five head
segments and their appendages are maxillipeds. Together
the head and first three thoracic segments form thecephalothorax. Each
thoracic segment of the cephalothorax bears a pair of maxillipeds but there are
few external indications of the original segmentation. Dorsally
and laterally the head and entire thorax are covered by the carapace.
thorax consists of eight segments, or thoracomeres, of which the first three are
part of the cephalothorax. The
five posterior thoracomeres are not fused with the head although dorsally they
appear to be because all are covered by the carapace. Ventrally,
however, the five segments can be seen to be independent of the cephalothorax
(Fig 2, 19-31). The
five independent thoracic segments are known collectively as the pereon and
their appendages are pereopods. Each
of these segments is a pereomere. The
pereopods are the five pairs of large, obvious walking legs (Fig 1, 19-31). The
name Decapoda (“ten feet”) alludes to these ten appendages.
where they are not covered by the carapace, the thoracomeres are easy to
distinguish (Fig 2, 19-31). The
first three, which bear maxillipeds, are fused to each other and to the head but
the posterior five, numbered 4-8 in Figure 2, are clearly independent of each
ventral surface of a typical arthropod segment is covered by an exoskeletal
plate known as asternite and
those of the thoracomeres form the ventral surface of the thorax (Fig 2, 19-31).
Figure 1. Dorsal
view of the blue crab, Callinectes
pereopods of the left side have been omitted. The
anterolateral teeth are numbered 1-8. The pereopods are numbered 1-5. m
= merus, c = carpus, p = propodus, d = dactyl. Crab20La.gif
carapace has two long lateral
spines and several strong teeth on
each anterolateral margin (Fig 1). The
posterior margin of the carapace is smooth or minutely beaded. The
lateral extensions of the carapace, known as branchiostegites,
enclose large branchial chambers which house the gills (Fig 19-36, 19-3A).
the anterior edge of the carapace, on each side of the midline are two shallow,
notched excavations. These
are the orbits, from
which the eyestalks protrude
(Fig 1). Each has acompound
eye at its distal end. Anteriorly
the cephalothorax bears a small anterior, median process, the rostrum (Fig
Figure 2. Ventral
view of a male blue crab. The
sternites of the thoracomeres are numbered 1-8. The
left coxae of the pereopods are numbered 1-5. The
abdomen is extended to reveal the abdominal appendages and penis. Crab21La.gif
the midline of the carapace, just posterior to the middle, is a short, shallow,
transverse groove (inconspicuous in Cancer). This
is the cervical groove and
it is the cross-bar of an H-shaped set of grooves (Fig 1). It marks the
approximate division between head and thorax.
the ventral surface locate the abdomen (Fig
2) flexed beneath the thorax (Fig 19-31). The
abdomen is also called the pleon,
its segments are pleomeres, and its appendages arepleopods. In
true crabs (i.e. Brachyura, such as Callinectes and Cancer)
the abdomen is a small segmented structure whose shape varies with sex and
mature females it is broad with convex sides and covers most of the posterior
ventral surface of the thorax. In
immature females the abdomen is a nearly equilateral triangle whereas the
abdomen of males is very narrow although it has a broad base. Determine
the sex of your specimen from the shape of the abdomen.
the abdomen so it is no longer flexed but points posteriorly from the thorax as
it would in a crayfish or shrimp. In
dorsal view most of its segments are easily seen and can be counted, especially
in females. The
small, triangular, terminal portion is the telson,
which is not a true segment. Most
malacostracans have six abdominal segments plus the terminal telson. In
female blue crabs the six segments are independent of each other and five of
them are visible, the first being hidden by the carapace. In
males, segment 1 is hidden under edge of the carapace, segment 2 is visible and
wide, and 3, 4, and 5 are visible but fused together and narrowed posteriorly
(Fig 2). Segment
6 is separate, slender, and has the telson attached to its end. (The
segments differs in other genera).
transparent, membranous intestine runs
along the ventral midline of the abdomen, under the thin membranous ventral
exoskeleton, and terminates at the anus on
the telson (Fig 2). It
may be filled with dark feces in which case it is easier to see. Press
its posterior end with a probe to extrude feces from the anus, thereby
confirming its position.
the ventral surface of the thorax is a median, longitudinal groove hidden by the
abdomen of the male occupies this groove and in females the gonopores are in its
walls. The female
gonopores are large
triangular openings in the sternites of the sixth thoracic segment, in line with
the third pair of legs. Male
gonopores are located at the tip of the inconspicuous penis on the last leg and
will be seen later.
the appendages without removing them from the animal. Each section of an
arthropod limb is known as an article. The
term segment is
reserved for the modular divisions of the body.
basic crustacean appendage is biramous. The
proximal article, the one that articulates with the body, is the protopod.
In many instances the protopod is divided into two articles, the coxa and basis. From
the protopod (or basis if the protopod is divided) arise two branches, or rami. The
lateral ramus is the exopod and
the medial is the endopod. If
one ramus is absent, the appendage is uniramous. If
both are present, it is biramous. Any
additional structures on the lateral side the limb are exites,
on the medial side they are endites.
Finally, an exite on the base of the appendage is an epipod.
appendages are numbered from anterior to posterior it is easier to study them in
reverse order, from posterior to anterior. Begin
with the pleopods, or abdominal appendages, and work your way forward through
the pereopods, maxillipeds, and mouthparts, to end with the antennae. Brachyuran
crabs have no uropods.
the abdomen again, look at its ventral surface, and find the abdominal
appendages (pleopods). Be
sure you see and study both sexes.
the abdomen itself, the pleopods are sexually dimorphic.
have only two pairs of pleopods and they are located anteriorly on the abdomen,
on segments 1 and 2 (Fig 2). Both
function in the transfer of sperm to the female during copulation. They
are hidden under the flexed abdomen, which must be extended to reveal them. The
long, curved, tubular first
pleopod is the gonopod. It,
not the penis, is the intromittent organ used to deliver spermatophores to the
female gonopore. The second
pleopod is much smaller and
functions as a piston to push spermatophores through the hollow core of the
have paired biramous pleopods on abdominal segments 2-5 and, as in the male,
they are hidden under the flexed abdomen which must be extended to reveal them. The
first article, or coxa (Fig 3), of a female pleopod is attached to the body by a
soft and flexible articulating
coxa is small and poorly calcified but the next article, the basis,
is large and conspicuous. Two rami,
the exopod and endopod,
arise from the basis. After
release from the gonopores, the eggs attach to the long setae of
the pleopods where they are ventilated by movements of the abdomen and the
Figure 3. The
pleopod of abdominal segment 5 of a female crab. Crab22L.gif
five pairs of pereopods, or walking legs, of the posterior thorax lack exopods
and thus are uniramous. They
are slender stenopods consisting solely of the endopod and protopod. They
have no exopod. In almost all swimming crabs (Portunidae) pereopod 5 is a
broad, oarlike swimming leg (Fig
1, 19-31). (The fifth pereopod of Carcinus
maenas, although a portunid, is not modified for swimming.)
the characteristic seven articles of
a typical thoracic malacostracan endopod (Fig 1). The
short, proximal coxa articulates
with the body (Fig 2). The
next two articles, the basisand ischium,
are fused together to form the basischium. The
suture marking the line of fusion is visible. The
basischium is followed successively by the merus, carpus, propodus,
dactyl and propodus of pereopod 5 are both flattened to form the paddle.
males the gonopore is
located at the tip of a long, thin, limp, transparent, colorless penis on
the proximal edge of the coxa of the fifth pereopod (Fig 2). (The
penis of Cancer is
a short, blunt papilla). The
penis fits into the groove of the gonopod to which it delivers sperm. The
gonopods, not the penis, are the intromittent organs.
can be voluntarily autotomized (= self cut) to escape predation, reduce blood
loss from a distal wound, or in response to physiological stress. A
special fracture plane is
located approximately at the line of fusion of the basis and ischium, where each
leg can be shed with a minimum of bleeding and trauma.
2-4 resemble pereopod 5 except that their distal articles are not oarlike (Fig
1 is the cheliped and
the pincer at its distal end is the chela. The
cheliped is larger and more robust than the other pereopods and is constructed
so that the dactyl is a movable
finger that opposes an immovable
finger protruding from the
arrangement creates a prehensile chela. Note the teeth on
Note the slight asymmetry of the two chelipeds. The
left, or cutter cheliped,
is smaller and its teeth are a little smaller and sharper. The
right, or crusher cheliped,
is a bit larger and has larger and slightly more rounded teeth. This
dimorphism may be reversed in some individuals and it is more pronounced in many
other crab species. (There
is little difference in the two chelipeds of Cancer
large opening in the carapace dorsal to the coxa of the cheliped is the inhalant
aperture leading into the branchial
chamber (Fig 2) where the
gills are located.
your crab is missing any of its legs, they were probably deliberately
autotomized by the crab. If
so, the end of the stump of the leg will be cleanly sheared at the fracture
plane and covered by a membrane. The
membrane is penetrated by a small hole through which pass an artery and nerve
(before autotomy). A
one-way valve over this opening prevents loss of blood after autotomy. Autotomy
avoids irregularly broken exoskeleton, and torn muscles. Regeneration
of an autotomized leg begins before the next molt.
three pairs of maxillipeds are the appendages of the first three thoracomeres. Unlike
other thoracopods they are biramous. Their
endopods are homologous to the endopods of the pereopods and are composed of the
same seven articles.
maxillipeds and other mouthparts overlie each other so only maxilliped 3 can be
seen at present. After
you study each appendage it should be moved aside to reveal the one beneath it
(anterior to it). Do
not detach the maxillipeds from the body.
at the crab en face, with
magnification as needed, and find the quadrate mouth
field including the mouth,
the area around it, and the mouthparts. The third
maxillipeds cover the mouth
field (Fig 2). They
are the appendages of the third thoracomere and together they resemble a pair of
doors protecting the mouth field and hiding the other mouthparts.
one of the third maxillipeds and note its mobility. It
is attached to the body by its protopod (Fig
protopod forms the medial border of the inhalant aperture and bears two rami. The endopod is
the wider of the two rami and its distal three articles form a small palp.
The endopod is
composed of the large proximal ischium followed
by the merus, carpus,propodus,
and dactyl. The exopod consists
of a long, narrow basal article and a multiarticulate flagellum used
to groom the face. At
the lateral corner of the protopod you will see a long setose flabellum extending
through the inhalant aperture into the branchial chamber. If
you move the maxilliped, the flabellum will move also, making it easier to
flabellum is used to clean the gills.
Figure 4. The
third maxilliped. Crab23L.gif
Second and First Maxillipeds
The second and first
maxillipeds are similar to
the third but are smaller (Figs 5, 6). They
are the appendages of the second and first thoracomeres respectively. Like
the third, they bear endopods, exopods, and lateral flabellae that extend into
the branchial chamber.
Figure 5. The
second maxilliped. Crab24L.gif
two exhalant apertures,
through which water exits the branchial chambers, are not so obvious as the
inhalant aperture but can be seen lateral to the first and second maxillipeds. Water
can exit these openings even when the third maxillipeds are closed over the
remaining five pairs of appendages belong to the head segments and comprise
three pairs of mouthparts and two pairs of antennae. Move
the maxillipeds aside to reveal the mouthparts.
Figure 6. The
first maxilliped. Crab25L.gif
posteriormost head appendage is the second
maxilla (Fig 7) lying
immediately anterior to the first maxilliped. It
is small, complex, and more delicate than the maxillipeds. It
has several parts, one of which is a large, lateral, flat, rectangular gill
bailer, or scaphognathite, which projects through the exhalent aperture
into the branchial chamber. The
bailer, which generates the respiratory current through the branchial chamber,
is the exopod of this appendage (Fig 19-38C). The
protopod has two flat, setose, bilobed endites and a vestigial endopod between
the bailer and the endites.
Figure 7. The
second maxilla. Crab26L.gif
maxilla (Fig 8) is even
smaller and more delicate than the second. It
has two endites and an endopod but no exopod.
Figure 8. The
first maxilla. Crab27L.gif
to the first maxillae are the large, hard mandibles (Fig
mandible consists of a heavily calcified protopod from which arises a small
protopod is divided into a medial cutting
surface and a large basal
region for the insertion of the muscles that operate the mandible. Only
the smooth, white cutting surfaces can be seen externally. Push
the mandibles back and forth and watch their motion. They
rotate on two movable articulations, or condyles, with the head skeleton and are
adapted for cutting, rather than grinding, food.
Figure 9. The
the two cutting surfaces apart and notice the soft protuberant upper
lip; a transverse fold of the body wall. Ventral
to the lip and between the mandibles, is the mouth. Gently
insert your blunt probe into the mouth to confirm its location.
pairs of antennae are small and may go unnoticed if they are folded out of sight
under the anterior edge of the carapace (Fig 1).
lateral pair is the second
crabs they lack exopods and are uniramous (Fig 10). Each
consists of a basal peduncle,
of three articles, whose proximal article is fused with the carapace and bears a
small, heavy, transverse ridge. Distally
the peduncle bears a slender flagellum of
Figure 10. The
first (right) and second (left) antenna of Callinectes
peduncular articles of each are numbered. The
inset is an enlargement of
the base of antenna 2 showing the nephridiopore and its operculum. Crab29L.gif
the short pronounced ridge running transversely across the first peduncle
article (Fig 10). Below
the ridge is a second, longer, ridge forming the dorsal border of the mouth
field. The external
opening of the nephridium, the nephridiopore,
is located in the depression between these two ridges. The
pore is covered by a movable calcified operculum. (In Cancer there
are no transverse ridges but there is a tiny oval operculum covering the
the tip of a needle under the ventral edge of the operculum and lift it to
demonstrate its mobility. In
living animals urine may escape when the operculum is lifted. The
operculum is best observed with magnification.
antenna (Fig 10) consists of
a triarticulate peduncle from
which arise two short inconspicuous flagella. The
basal article of the peduncle is broad and swollen and fits in a socket in the
anterior body wall. It
contains a statocyst for the detection of gravity (Fig 19-7B). The
statocyst is an invagination of the exoskeleton containing a statolith resting
on sensory setae. The
basal peduncular article is not fused to the carapace. The
first antennae fold on themselves and fit neatly into a recess in the carapace.
small, pointed, median rostrum extends
anteriorly between the bases of the two first antennae (Fig 1).
two short, thick eyestalks,
which are not segmental appendages, are located on the anterior edge of the
head in the orbits (Fig
large compound eye,
located at the end of each eyestalk is composed of hundreds of independent
photoreceptive units, or ommatidia. The
external manifestation of each ommatidium is its cuticular lens.
the crab so its dorsal side is up. Insert
the tip of a heavy scissors beneath the lateral, posterior edge of the carapace,
dorsal to the coxa of the fifth leg, and make a cut around the periphery of the
carapace on its dorsal surface
(Fig 11). Be
careful that you cut only the heavy calcified exoskeleton and not the organs
beneath it. Keep
your scissors about 5 mm from the edge of the carapace and cut completely around
a scalpel to separate it (by scraping, not cutting) from the underlying tissues. Carefully
remove the carapace, in pieces if necessary, with minimal disturbance to the
thin, dark body wall, which is little more than the epidermis,
lies immediately beneath the carapace and as much of it as possible should be
removed with the carapace. The exoskeleton and epidermis are the
body wall, as there is no musculature, connective tissue, or peritoneum in it.
two small, digitiform, calcareous processes on the inner surface of the carapace
almost exactly in its center. These
are apodemes for
the origin of muscles running to the gut. These
muscles must be disconnected to remove the carapace.
The hemocoel is
the large space in which the viscera lie. The
coelom is present only as small spaces associated with the gonads and nephridia.
remove most of the remaining epidermis (i.e. body wall) without damaging the
underlying tissues. It
may help to flood its surface with water to facilitate its removal. This
is a tedious task but must be done with care as some of the crab's organs adhere
tightly to it.
Make a wholemount of a small piece of epidermis being sure the tissue is flat
and not folded. Examine
the preparation with the compound microscope. The
pigment in the body wall is contained in conspicuous chromatophores (Fig
are irregular, star-shaped multicellular organs containing red, blue, or white
pigment granules. Red
and black can be found easily but the white chromatophores are harder to see
because the pigment is not so vivid.
pigment can be dispersed into the lobes of the chromatophore to increase its
visual effect or it can be concentrated in the center to minimize it. <
a preliminary examination of the hemocoel and its viscera to locate major
structures for use as landmarks. If your specimen is a mature female, the orange ovaries may
cover and obscure other structures. The
smaller, white testes of
the mature male do not obscure other structures. It
may be necessary to remove the ovary (but nothing else)
from one side in order to see the stomach and digestive ceca beneath.
The stomach (=
proventriculus) is a large, bulging, transparent, thin-walled sac lying dorsally
on the midline in the anterior thorax (Fig 11, 19-35). The digestive
ceca are large, soft,
amorphous, yellow or greenish organs occupying the periphery of the dorsal
may be completely obscured by the ovary in mature females. (The
digestive ceca of Cancer are
gray-brown and consist of abundant small, fingerlike papillae.)
Figure 11. Dorsal
dissection of a mature male crab. The
digestive cecum, gonads, and gills have been removed from the left side.
large, triangular, firm, beige or greyish mass of gills occupies
the branchial chamber in the space medial to the lateral spine (mostly posterior
to the lateral spine in Cancer). The
gills are sometimes called "dead man's fingers".
triangular mass of gills is covered by a very thin, transparent membrane which
you should avoid damaging. Posterior
to each gill chamber is a heavy endoskeletal plate called theflanc that
covers the powerful swimming muscles of pereopod 5. These
muscles are the "backfin" crabmeat of the seafood industry. (In Cancer the
skeletal plate enclosing the muscles of the last leg is not markedly larger than
those of the other legs). The
soft, white or gray heart lies
on the midline posterior to the stomach and between the flancs.
more careful study of the organ systems can now be undertaken. Systems
are considered in the order in which they are exposed by the dissection. The
internal organs of crabs have little connective tissue and are very delicate so
that some organs appear shapeless and without definite structure.
opaque white or gray heart (Figs
11, 12, 19-39) lies in a hemocoelic space known as the pericardial
unlined, blood-filled region in the hemocoel is not a coelom and should not be
called a pericardium, although it often is. The
heart has three pairs of large ostia,
two dorsal and one lateral, through which blood enters the heart (Fig 11,
19-35). The heart is suspended by numerous elastic suspensory ligaments that run
to the surrounding tissues. These ligaments are stretched during systole
(contraction) and then return to their original length when the heart muscles
relax during diastole. This
restores the heart to its original size and volume and results in the influx of
blood through one-way valves in the ostia into the heart lumen.
heart of living specimens may be beating. Contractions
of the heart pump blood anteriorly, ventrally, and posteriorly via a set of
seven arteries (Fig 12, 19-39). The
arteries are difficult to observe in fresh specimens but some, or all, of them
are usually visible in preserved animals. The
arteries branch repeatedly until, by the time they reach the tissues, their
diameter is that of capillaries.
blood leaves the capillaries and bathes the tissues. It
then flows into the hemocoel, passes through the gills, and then drains back
into the cardiac hemocoel. When
the heart relaxes, the valves of the ostia open and admit blood to the heart
contraction the valves close and blood enters the arteries.
blood, or hemolymph, contains the respiratory pigment hemocyanin, which is
colorless when deoxygenated and pale blue when oxygenated. The
pigment is in solution in the hemolymph.
the heart is beating, inject a solution of toluidine blue in seawater through an
ostium into its lumen and watch the dye enter the major arteries. Trace
the route of the arteries as far as possible. <
arthropod gut consists of an anterior foregut, middle midgut, and posterior
foregut and hindgut are derived from ectoderm and are lined with exoskeleton
that is shed with each molt. The
midgut is an endodermal derivative and has no cuticular lining. The
foregut comprises the mouth, esophagus, and stomach. The
crab midgut consists of the tubular midgut itself and several diverticula
including the digestive ceca and three midgut ceca. All
digestion and absorption occur in the midgut and its derivatives. The
hindgut consists of the tubular intestine, rectum, and anus.
large yellowish digestive
ceca (= midgut glands,
digestive glands, or hepatopancreas) are diverticula from the midgut (Fig 11). They
connect with the midgut near its junction with the stomach but the connections
are difficult to observe. The
digestive ceca extend along the anterior edge of the carapace, over the
branchial chambers, and in the spaces around the heart and gonoducts. The
ceca secrete hydrolytic enzymes into the stomach, absorb the products of
hydrolysis, and store food reserves.
The stomach is
the largest and most conspicuous part of the gut (Fig 11, 19-34, 19-35). It
is an exceptionally complex structure whose walls bear some 40 calcareous
ossicles and 80 muscles. It
is divided into a large, dorsal cardiac stomach (or anterior chamber) and a
smaller, ventral pyloric stomach (or posterior chamber).
stomach is the large
balloon-like structure in the anterior thorax. It
lies dorsal to the mouth to
which it is connected by the short esophagus (Fig
walls contain the ossicles of the gastric
mill, used for grinding food. The
ossicles are part of the exoskeleton and, being exoskeletal, are shed and
replaced with each molt. The
mandibles cut food into small pieces which are passed to the gastric mill for
trituration and mixing with hydrolytic enzymes from the digestive ceca.
pyloric stomach is the much smaller ventral region of the stomach. It
lies posterior and ventral to the cardiac stomach and is hidden by it.
on either side of the mouth and esophagus to find the white calcareous internal
part of the mandible extending into the anterior body cavity (Fig 9). The
four powerful muscles that operate the mandible insert here, three of them by
calcareous white tendons. Gently
push the mandible back and forth and watch the response of the cutting edge. Two
muscles, the lateral adductor and posterior adductor, move the cutting surfaces
together (Fig 9, 11). Two
other muscles, the abductor minor and abductor major, move the surfaces apart
examination of the remainder of the digestive system and then return to this
part of the exercise AFTER study of the reproductive system. At
that time, remove the reproductive organs in the vicinity of the heart if you
have not already done so. GENTLY
lift the posterior end of the cardiac stomach and look beneath it for the small
pouchlike pyloric stomach (14,
19-35, 19-34). Do
not damage the muscles and other tissues in this area. The
walls of the pyloric stomach bear the abundant ossicles and fine setae of the
filter press that is used to sieve particles from the liquid in the stomach. The
liquid, which contains the products of hydrolysis, is sent to the digestive ceca
the proventriculus is elevated, find the delicate, transparent midgut running
posteriorly from the posterior chamber under the heart to join the hindgut in
the anterior abdomen (Fig 11).
hindgut, or intestine,
runs along the ventral midline of the abdomen to empty through the anus located
on the ventral surface of the telson (Fig 2).
your scissors, make a transverse cut across the top of the stomach and look
the ossicles of
the gastric mill, find the pyloric stomach on the floor of the posterior half of
the cardiac stomach. Find the filter
press in the pyloric
the tip of the probe into the mouth and use it to locate the opening of the
esophagus into the stomach.
respiratory system consists of the gills located
in the two lateral branchial
chambers (Figs 1, 11,
large chambers occupy the pointed lateral sides of the body and are bounded on
all surfaces by the skeletal branchiostegite of the carapace. The
branchiostegite is a double fold of body wall enclosing the branchial chamber. It
is the lateral carapace. Of its two layers, the outer is heavy and calcified and
part of it has been removed. The
inner layer is thin, uncalcified and unsclerotized and is still intact, covering
the gills. This
transparent body wall lying over the gills is no more than a thin chitinous
membrane investing the dorsal surface of the branchial chamber. It
is almost invisible but it is all that separates the branchial chamber (which is
filled with seawater) from the hemocoel (which is filled with blood). Find
and remove this thin, transparent sheet from the surface of the gills. The
branchial chamber is now open and the gills are exposed for study.
are eight gills on each side of the body but two of them are small and easily
gills are exites of thoracopods. Each
of the eight gills consists of a long central
axisto which are attached, on opposite sides, two rows of very closely
spaced flat branchial lamellae (Fig
a microneedle to separate two adjacent lamellae from each other and look at them
with magnification. Together the branchial lamellae provide an immense surface
area for gas exchange. The
lamellae are covered by cuticle which is molted periodically along with the rest
of the exoskeleton.
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.
crabs and shrimps, including Callinectes,
have lamellar (= phyllobranchiate) gills in which the respiratory surface
consists of numerous flat lamellae radiating from a central axis (Fig 19-37E,F). Look
at the cut surface of the gill axis to find the blood channels, cut in
cross-section, that extend the length of the gill axis (Fig 19-37E). These
include the afferent channel that delivers unoxygenated blood to the gill and
the efferent vessel that drains oxygenated blood away from the gill.
gills projecting into the branchial chamber divide it into dorsal and ventral
regions (Fig 19-36). Water
flows in the inhalant aperture to the ventral inhalant
chamber (= hypobranchial
chamber), then across the gill filaments into the dorsal exhalant
chamber (= epibranchial
then exits via the exhalant aperture.
a blunt probe into the inhalant aperture at the base of the cheliped and observe
that it enters the ventral inhalant chamber, below the gills. The
probe can be pushed gently upward through the curtain of gills into the dorsal exhalant
chamber above the gills thus
tracing the route taken by the respiratory water current through the gill
chamber (Fig 19-38). Now
insert your probe into the exhalant aperture and note that it enters dorsally,
above the gills, in the exhalant chamber.
normally flows into the crab on its ventral side, up through the gills and then
out dorsally (Fig 19-38). The water flow is generated by rhythmic undulations of
the gill bailer. Reversing
the beat of the bailer reverses the direction of flow over the gills. This
backflushes the gills to clean them or to respire at the surface of poorly
oxygenated water. Crabs
attempting to respire out of water or in very shallow water may blow bubbles
from the exhalent apertures beside the mouth if they do not reverse the
direction of flow. Some
crabs, such as the portunid lady crab,Ovalipes, burrow backwards into
sand and accordingly employ reversed
water flow through the branchial chamber. This avoids fouling the intake with
sand and also allows the crab to utilize the posterior outflow of water to
loosen the sand ahead of digging.
living, unrelaxed specimens are available, place one in an 8" finger bowl of
clean sea water. Tap
water can be used if seawater is unavailable. The
fingers of the chelipeds must be bound with elastic bands so the crab cannot
a plastic pipet with 1% toluidine blue/seawater solution or other non-toxic dye. Remove
the crab from the dish. Insert
the tip of the pipet into the inhalant aperture
at the base of the cheliped and deliver 2-3 ml of dye deep into the branchial
the crab back into the dish of water, watching its mouth field as you do. The
flow of water through the branchial chamber will sweep the dye out of the
chamber via the exhalent aperture. Watch
the flow of colored water from the animal and determine the point at which it
leaves the body. Does
this correspond with the position of the exhalant apertures? Does
the dye emerge from both exhalent apertures or only one? <
the gills of your dissected specimen and look at the floor of the branchial
you will see the narrow, setose flabella of
the second and third
maxillipeds in the inhalant
chamber (Fig 11, 19-36). The
flabellum of the third maxilliped is the longer of the two. With
forceps wiggle the two maxillipeds in turn and watch their flabella move. These
flabella are moved by the activity of their respective maxillipeds.
in the exhalant chamber along the anterior edge of the base of the gills to find
the flabellum of
the first maxilliped. This
flabellum has its own muscles and operates independently of its maxilliped. Wiggle
the first maxilliped and see that its flabellum does not respond as did those of
the other two maxillipeds. The
flabella sweep over the surface of the gills and keep them clean. The
second and third maxillipedal flabella clean the inhalant side of the gills
whereas the first maxillipedal flabellum cleans the exhalant side.
flabella also participate in circulating water through the branchial chamber. The
gill bailer of maxilla 2 can be seen beside the flabellum of maxilliped 1. The
gill bailer beats to move water out the exhalant apertures and has the chief
responsibility for generating the respiratory current through the branchial
spite of the activity of the flabella the gills are sometimes fouled. The
tiny stalked barnacle, Octolasmis (2mm)
lives attached to the gill lamella of several species of crabs. In
this position these filter feeders take advantage of a protected habitat in the
branchial chamber through which there is a constant flow of food and oxygen rich
water. You may see some
of these on your crab’s gills. A small, orange, nemertean worm, Carcinonemertes
carcinophila, also inhabits the branchial chamber of blue crabs.
morphology and appearance of the reproductive system vary markedly depending on
sex and degree of maturity of the specimen. The
following accounts are of mature individuals and if your specimen is immature
you may not find all of the structures mentioned.
two long, paired, indistinct, white or grayish, testes lie
dorsally in the anterior body where they may be difficult to distinguish from
the digestive ceca beneath them (Fig 11). Each
testis is a translucent convoluted tube that begins laterally near the base of
the lateral spine and extends anteriorly and medially. It
lies on top of the digestive ceca and parallels the anterolateral border of the
the stomach it turns posteriorly and parallels the border of the stomach and
approaches the midline.
the midline the testis becomes the complex, coiled and looped vas
deferens of many regions and
colors (white, pink, greenish). In Callinectes,
the proximal vas deferens exits
the testis and is located near the midline posterior to the stomach. It
is a highly convoluted, small-diameter tubule wound on itself to form a globular
white color is due to white spermatophores which are formed here of sperm from
the testes. The
spermatophores look like tiny white eggs.
proximal vas deferens connects with the large, conspicuous, pink middle
vas deferens which lies
beside the lateral border of the stomach ventral to the testis. In
mature specimens this is the easiest part of the male system to see. It
is large, pink, and glandular in mature individuals. A pink jelly plug that will
be transferred to the female during copulation is formed here.
middle vas deferens turns ventrally at its anterolateral end and doubles back
underneath itself to become the distal
vas deferens. This
large, thick, greenish, translucent, convoluted tube is not obvious, in spite of
its size, because it is ventral to the proximal and middle vasa deferentia and
is obscured by them. Part
of the digestive ceca surround the vas deferens and hide them. You
must remove this part of the ceca if you wish to see the distal vas deferens. The
distal vas deferens makes several loops ventral to and posterior to the middle
vas deferens and then narrows dramatically, ceases its coiling, and becomes a
slender tube extending obliquely to enter the penis at the base of the last leg.
a few spermatophores from the proximal vas deferens and place them on a slide
with seawater or isotonic saline but do not add a coverslip yet. Look
at the spermatophores with the dissecting microscope. They
are ovoid in shape and are packed with spermatozoa. Affix
a coverslip and withdraw enough of the water from beneath it so that some of the
spermatophores rupture from the pressure of the coverslip. Examine
the contents of a ruptured spermatophore with high power of the compound
microscope (400X). The
spermatozoa are small, irregular, unflagellated cells with short cytoplasmic
the mature female, the orange ovaries may
be small or so large they obscure other organs in the hemocoel (Fig 12, 19-35). In
immature individuals, the ovary is beige or white and much less conspicuous. The
right and left ovaries are connected across the midline by a narrow isthmus and
each consists of anterior and posterior horns. Together
the ovaries and the connecting isthmus form an "H" (Fig 12).
oviducts exit the ovary and connect with the female gonopores on the sternite of
thoracomere 6. The
distal region of the oviduct is the seminal
receptacle, part of which has a hard chitinous wall (Fig 12). Each
of the two receptacles is located a little lateral to the midline between the
stomach and heart along the posterolateral border of the stomach next to the
gill mass. In the male the middle vas deferens occupies the equivalent position.
size and color of the receptacle varies and may be quite large, hard, and pink
for a brief period following copulation when the sperm mass and its large, pink,
jelly plug are present. Later
the receptacles shrink again and turn white as the jelly is absorbed. Sperm
are retained in the seminal receptacle where they will later be used to
fertilize the eggs. Copulation
occurs while the ovary is still white and immature. Later
it will turn orange and expand greatly in size as orange yolk accumulates.
blue crabs mate once and usually spawn twice, using the sperm from the single
mating for both spawns. Female
crabs mate following their terminal molt and the male crab carries his mate
about awaiting this event. After
the female molts, the male turns her on her back and inserts his gonopods into
the female gonopores (Fig 19-47B). He
deposits sperm in her seminal receptacles, which lie just inside the gonopore. Sperm
remain here while the female migrates downstream in the estuary. Eggs
are fertilized as they move through the oviduct to the exterior. Almost
two million of them may be shed and attached to the setae of the pleopods and
there begin their development.
Figure 12. Dorsal
dissection of a fertilized but sexually undeveloped female crab. The
seminal receptacle is filled with sperm but the ovary is small and undeveloped. Notice
the "H" shape of the gonad. Crab31La.gif
excretory system consists of two soft, grayish or pale greenish-white antennal
glands located inside the anterior wall of the cephalothorax behind the second
antenna (Fig 11). They
may be difficult to find. The
glands are saccate nephridia with an end sac derived from the coelom and a
tubule and labyrinth derived from its coelomoduct (Fig 19-6B). Ultrafiltration
of the blood occurs across the walls of the end sac and the filtrate is modified
by the labyrinth. The
final urine is stored in two very large, thin-walled, transparent bladders which
empty to the exterior via the nephridiopores on the peduncle of antenna 1. The
walls of the bladder are very thin and are not easily recognized, even when
intact and undamaged.
antennal glands are effective osmoregulatory organs and blue crabs are tolerant
of a wide range of salinities. The glands play little or no role in nitrogen
excretion, most of which occurs across the surfaces of the gills.
nervous system is transparent and difficult to visualize in fresh material. It
is slightly easier to see if a pipet of 99% isopropanol or ethanol is applied to
the area inside the head and thorax. After
a few minutes the nervous system will become opaque and white.
Figure 13. The somatic nervous system of the cancrid
crab, Cancer. Redrawn from
Pearson, 1908. crab50.gif
nervous system of brachyuran crabs is highly cephalized into a brain and a large
thoracic ganglion (Fig 13). The brain is
located dorsally in the head immediately posterior to the rostrum, between the
two eyestalks, and on the midline. It
lies under a layer of muscle and connective tissue that must be removed before
it can be seen. Emerging
from the brain are optic nerves to the compound eye, oculomotor nerves to the
eyestalk muscles, antenna 1 and antenna 2 nerves to the two pairs of antennae,
and a tegumentary nerve to the anterior integument (Fig 13).
long circumesophageal connectives leave the brain to run posteriorly and
ventrally around the sides of the esophagus. Well
posterior to the esophagus they join the thoracic
ganglion which is formed of
the coalesced paired ganglia of all thoracic and abdominal segments. Paired
nerves radiate from this ganglion to each thoracic appendage and a single
abdominal nerve extends to the reduced abdomen and its appendages (Fig 13). The
thoracic ganglion is obscured by connective tissue and muscle which must be
removed to reveal it. It
lies at the intersection of the midline and a line connecting the seventh
anterolateral teeth of the two sides of the carapace (Fig 1). Segmental
nerves to the thoracic segments exit this ganglion as does a single, median
nerve to the abdomen.
the large, movable transverse apodeme extending
from one eyestalk to the other. It
is a rigid connection between the two that insures the two eyes will act in
the apodeme and observe the response of the two eyestalks.
DE . 1982. Shrimps,
Lobsters, and Crabs. New
Century, Piscatawaty, New Jersey. 242p.
DE, Mantel LH(eds.). 1983. The
Biology of Crustacea, vol. 5: Internal Anatomy and Physiological Regulation . Academic
Press, New York. 471p.
WK . 1890. Handbook
of Invertebrate Zoology. Bradlee
Whidden, Boston. 352p.
(ed) 1950. Selected
Invertebrate Types. Wiley,
New York. 597p.
Cochran DM. 1935. The
skeletal musculature of the blue crab, Callinectes
sapidus Rathbun. Smithson.
Misc. Coll 92(9):1-76.
LE. 1947. Anatomy
and histology of the male reproductive system of Callinectes
sapidus Rathbun. J.
JH . 1967. The
biology of the shore crab, Carcinus
maenas (L.). Field
FW, Humes AG (eds.). 1992 . Microscopic
Anatomy of Invertebrates vol. 10 Decapod Crustacea. Wiley-Liss,
New York. 459p.
J-S . 1978. Laboratory
Manual for Invertebrate Zoology Emphasizing Marine Forms. Hwong,
Los Alamitos, Calif. 152p.
PT. 1980. Histology
of the Blue Crab, Callinectes sapidus. A
Model for the Decapoda. Praeger,
New York. 440p.
J, Bucherl W. 1940. Contribuicao
ao estudo da anatomia e fisiologia do genero Callinectes (Crustacea
Decapoda, Fam. Portunidae). Arq.
Zool. Sao Paulo, 1:153-217.
J. 1908. Cancer. Liverpool
Marine Biology Committee Mem. 16:1-217, 13 pls.
R, Cronin E . 1950. The
general anatomy of the blue crab, Callinectes
sapidus Rathbun. Chesapeake
Biological Lab. Pub. 87:1-38.
Ruppert EE, Fox RS. 1988.
Seashore animals of the Southeast. Univ. South Carolina Press, Columbia. 429 pp.
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
RI . 1978. The
midgut caeca and the limits of the hindgut of the Brachyura: A clarification. Crustaceana
GF. 1978. The
Biology of Crabs. Van
Nostrand Reinhold, New York. 202p.
WW. 1976. Beautiful
Books, New York. 304pp.
AB. 1978. The
swimming crabs of the genus Callinectes (Decapoda:
AB. 1974. Shrimps,
Lobsters, and Crabs of the Atlantic Coast of the Eastern United States, Maine to
Institution Press, Washington. 550pp.
Slides and coverslips
Living or preserved blue crab
Large dissecting pan (about 20 cm long)
99 % isopropyl alcohol
Rubber rings made by cutting cross sections of surgical
NaCl, 0.086g KCl, 0.142g CaCl 2,
0.166g MgCl 2•6H 2O
per 100 ml water at pH 7.3-7.5 (Pyle & Cronin, 1950).
Living Maryland blue crabs can be ordered from Chesapeake
Crab Connection, www.ordercrabs.com . Sizes
are 5–7+ inches with prices, per dozen, varying from $22-$75 depending on size.
Shipping and handling vary with quantity and distance but will usually be at
least $50. The
web site will calculate the cost for you.