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This class of arthropods includes the shrimps, water fleas, copepods, barnacles, crayfish, lobsters and the crabs. Many are marine but there are also well represented in freshwater habitats. They range in size from microscopic to very large forms like the giant crabs which may reach a length of 3.6 m.
The
crayfish will be studied as a representative Crustacean. Crayfishes are common
in streams and rivers, and even in some lakes and marshes, in many parts of
Europe, Asia, North and South America, and Australia. In some areas they are
extensively harvested as a food source. The most favourable factor to their
presence is a sufficiently high concentration of dissolved calcium. Crayfish
are very sensitive to acid conditions and are not found in acidic environments.
They are therefore, extremely susceptible to acid rain and have disappeared
from many lakes. During the day they avoid the light by hiding in crevices or
in burrows which they excavate themselves. Activity begins in the evening when
they search for their food, which consists of almost anything organic, animal
or vegetable, dead or alive. They may also be cannibalistic. Snail-shells and
calcareous algae are often eaten for their calcium content. In winter they burrow
deeply, and remain inactive during the coldest weather. Mating takes place commonly
in autumn, the eggs being carried glued to the pleopods (or swimmerets) of the
female until the spring. After hatching the young crayfish still lack some of
the abdominal appendages, and for some further time they maintain their grip
on the pleopods by means
of the peculiarly hooked tips of their chelae.
Examine the external views of the crayfish:
Anteriorly, the body is enclosed in a cephalothoracic shield. The head is the region anterior to the cephalic groove, and it is prolonged anteriorly into a pointed rostrum, lateral to which are the two stalked eyes. The thorax is covered by the carapace, and its segmentation is shown by the presence of pairs of limbs, of which the cheliped and walking legs are the most prominent. The abdomen comprises the six posterior segments and the telson. The anus is the longitudinal slit located beneath the telson.
The first of the head appendages are the relatively short biramous antennules. The long antennae are obvious on the adjacent segment. The posterior head appendages lie obscurred by the maxillipede (anterior thoracic appendages). The thoracic segments are fused to the carapace which overhangs on each side to enclose a gill chamber.
The posterior thoracic appendages are the cheliped and pereiopods (walking legs). Note the jointed appearance and the shape of these appendages. The abdominal appendages consist of five pairs of pleopods (or swimmerets) and one pair of uropods. The pleopods of the female are all similar (and serve to carry the eggs), except that the first pair is reduced. In the male the 3rd and 5th pleopods are similar to those of the female, but the 1st and 2nd are modified. The first is reduced to an unjoined grooved rod, the second has a portion stiffened (they both serve in copulation). The 6th appendage, the uropod, together with the telson forms the tail-fin (used in rapid backward swimming to escape danger).
Locate the mouth lying between the mandibles. Each mandible has attached to it a long tendon which extends dorsally to a mandibular adductor muscle which attaches to the dorsal part of the carapace. The muscle tendon arrngement works the mandibles. The mouth leads into a short esophagus which in turn enters the large cardiac stomach (anterior and has a grinding function); this empties into a smaller, posterior pyloric stomach which directly enters the intestine. In the thoracic cavity just anterior to the mouth are two flattened glands, the antenary glands which have an excretory function.
Examine the crayfish after a dorsal piece of the carapace has been removed
The window created exposes the heart, posteriorly, the creamy colored tops of the digestive glands and the top of the stomach at the anterior end.
Next make a cut through the carapace on one side from the anterior end of your window ventrally to the base of the carapace. At this point the entire side of the carapace may be gently removed exposing the branchial chamber containing the gills. Note that the gills curve forward along the direction of the water current which enters the chamber posteriorly. Notice that each of the gills is attached at the base of a leg.
Examine the crayfish with the side carapace removed
Carefully remove each leg at the base, along with its attached gill (image).
After this is completed the inside of the branchial chamber will still be separated from the internal organs by a thin, clear piece of exoskeleton. This should be carefully removed to expose the internal organs.
The internal organs lie in the haemocoel. The most prominent organ will be the digestive gland. The digestive gland is shaped like a J lying on its back. In the postero-dorsal part of the thorax locate the heart which is located in a large pericardial sinus which receives blood laterally from the gills. Blood enters the heart via openings called ostia and is distributed anteriorly and posteriorly through arteries.
At this point make a drawing of the lateral view of the crayfish.
Next, remove the exoskeleton covering each of the abdominal somites. The bulk of the abdomen is made up of the very large abdominal muscle. In the dorsal surface of the muscle is a groove, in which the intestine runs posteriorly to the anus.
Examine the crayfish abdominal region
