ANIMALS WITH JOINTED FEET-THE ARTHROPODS 



207 



lobster and shrimp are the parts usually 

 eaten. The fresh-water crayfish (Fig. 11-5) 

 and other Crustacea are generally not eaten 

 in this country, although in certain parts of 

 the world they are considered excellent 

 food. In addition, the Crustacea play an 

 important role in the food chain of fishes 

 (see p. 95). There is a stage in the de- 

 velopment of all fish when they must feed 

 on some form of Crustacea; this may be 

 the larval stage of some larger form, such 

 as the crayfish, or it may be a minute adult 

 crustacean, such as cyclops or daphnia 

 (Fig. 11-3). If it were not for these small 

 animals, fish would never get through the 

 early part of their life. Some Crustacea, on 

 the other hand, act as intermediate hosts for 

 certain dangerous parasites of man, such 

 as the human lung fluke, Paragonimus 

 westermani. On the whole, the group is 

 important both economically and biologi- 

 cally. 



Let us consider a representative member 

 of the Crustacea in some detail. The lobster 

 and crayfish are excellent examples and be- 

 cause of their universal distribution are 

 readily available for study. 



The lobster and the crayfish 



These two closely related animals live in 

 different environments, the lobster in the 

 sea and the crayfish in fresh water, yet their 

 bodies have much the same appearance. In 

 fact, a description for one also fits the other 

 fairly well. This discussion is confined 

 largely to the lobster, although frequent 

 references will be made to the crayfish. 



The lobster is a bottom dweller, spending 

 its life seeking food, which consists of other 

 Crustacea, small fish, and mollusks. It feeds 

 as a scavenger whenever it finds dead ani- 

 mals on the ocean floor. It is a secretive ani- 

 mal, hiding by day in any cavern that 

 affords enough space for its body, and 

 sallying forth at night, crawling forward 

 along the sea bottom, alert to any stimuli 

 that may mean food is in the vicinity. If in 

 danger of attack by other predators, it can 



swim backward with darting speed by 

 powerful strokes of its abdomen. In so 

 doing, it stirs up the mud at the bottom, 

 thus forming a "smoke screen," which con- 

 fuses the intruder and allows the lobster 

 an opportunity to gain distance to a safe 

 place. The crayfish employs the same meth- 

 ods in defending itself, and it seeks the 

 same type of food in much the same 

 manner. 



Structure. The animal is enclosed in a 

 chitinous exoskeleton, containing consider- 

 able quantities of lime and sclerotin which 

 make the skeleton rather heavy and bulky, 

 but a very excellent armor plate (Fig. 

 11-8). Since the animal is suspended in 

 water, most of its weight is taken care of 

 by buoyancy, and it is still a very agile 

 creature. The chitin thins out at the joints, 

 allowing maximum flexibility. The anterior 

 portion of the body is covered by the cara- 

 pace, and each posterior abdominal seg- 

 ment by an arched dorsal tergum, two 

 lateral pleura, and a ventral sternum. Tiny 

 holes perforate the entire skeleton, being 

 particularly numerous in the appendages 

 and tail region. Set into these are bristles, 

 which make the animal extremely sensitive 

 to its surrounding world through tactile 

 stimulation. 



The appendages of the lobster or crayfish 

 demonstrate a very interesting series of 

 adaptations and modifications for a particu- 

 lar mode of life (Fig. 11-9). There are nine- 

 teen pairs of appendages in all, one pair on 

 each segment. The antennules and antennae 

 are modified for tactile and chemical stimu- 

 lation; the mandibles, or jaws, for chewing; 

 the next five, maxillae and maxillipeds, 

 chiefly for food manipulation; the next pair, 

 the enormous chelipeds, for grasping food 

 and for defense; the next four for walking; 

 and the last six for swimming and various 

 other functions. All of these appendages, 

 with their varietv of form and function, 

 come originally from a simple appendage 

 with a single function, namely, locomotion 

 (Fig. 11-10). 



