CHAPTER XX.
SOME CRUSTACEANS, MOLLUSCS,
AND WORMS.
65. SOME COMMON CRUSTACEANS.
1. The crayfish and lobster.—(a) Habits.—Readers in limestone districts will probably be able to find crayfishes in the streams, and the habits of the animals in their natural surroundings should be observed and described. Other readers will be able to obtain live specimens from dealers.[35]
Place the animal on the table or desk. Notice that the body consists of an anterior, unsegmented portion, the cephalothorax (corresponding to the head plus the thorax of an insect), and a posterior, jointed abdomen. Watch the movements of the stalked eyes, the feelers and legs; allow the claws of the largest pair of legs (the “pincers”) to grasp a pencil. Put the crayfish in a white dish, with an inch or two of water, and by means of a pipette discharge a few drops of water containing some suspended colouring matter, such as carmine or indigo, near the point of attachment of the last walking leg. Describe the movements of the coloured water. When the animal is startled, notice the sudden vertical flexure of the abdomen, by means of which the body is pulled backward in the water. Feed the crayfish with meat or worms, and try to see the action of the jaws.
(b) External characters.—Kill the crayfish instantaneously by dropping it into boiling water. Compare the animal with a cockroach (p. 349). Examine more closely the stalked eyes, the two pairs of feelers, the four pairs of walking legs and the single pair of large pincers, and the fusion of the head and thorax to form the cephalothorax. Count the segments of the abdomen.
(c) The appendages.—Examine the ventral surface of the abdomen, and notice that each segment, except the last, bears a pair of small jointed organs; these are called swimmerets. Remove one of the pair carried by the last segment but two, examine and draw it; make out that it consists of a stalk and two branches. Notice that the branches of the swimmerets of the last segment but one are expanded, and form, with the last segment, the tail fin.
Study the appendages of the cephalothorax from behind forwards. They consist of (i) four pairs of legs used for walking, and a larger pair of pincers; (ii) three pairs of foot-jaws or maxillipedes; (iii) two pairs of maxillae; (iv) one pair of mandibles; (v) two pairs of feelers.
(d) The gills.—With strong scissors cut off the side part of the exoskeleton of the cephalothorax, and notice the plume-like gills in the gill-chamber thus laid open. Move the adjoining legs, and see that some of the gills also move.
2. The crab.—Obtain a crab and compare it, point by point, with the crayfish, or lobster. Notice the great width of the shell of the cephalothorax; much of this width is due to the gill-covers, which stand out from the sides of the true body. Look for the opening of the gill-chamber at the base of the pincers. Make out the stalked eyes (in sockets), the two pairs of feelers, and the five pairs of legs. Notice how the animal runs; in what respects does the method differ from the gait of the crayfish? Examine the ventral surface of a dead specimen, and notice that the abdomen is tucked under the cephalothorax. Stretch out the abdomen; compare it with that of the crayfish, and count the segments. Notice the absence of the tail fin; it is not required, as the crab does not swim. Extend the abdomen, and make drawings of the animal (i) from above, and (ii) from below, and label the parts. Carefully remove the foot-jaws and the true jaws, and compare them with the corresponding appendages of the crayfish. With strong scissors cut off one of the gill-covers, and examine the gills. Observe that in the crab the gills are not in any case attached to appendages; they all spring from the sides of the body-wall.
Tabulate the respects in which you have found the crayfish and crab (i) to agree with, (ii) to differ from, insects.