GENERAL ZOOLOGY 



banks of ponds and streams. Some forms burrow for considerable distances 

 into the banks, or into the soil of wet meadows far from open bodies of water. 

 They construct entrances and air holes that appear as chimney-like masses of 

 mud brought up and deposited around the openings. The burrow usually 

 terminates in a chamber below or near the waterline. 



In moving about upon the bottom of a stream or pond, the crayfish walks 

 slowly forward with its great claws held in front of the body. Its common es- 

 cape reaction is to dart backward through the water with great rapidity, pro- 

 pelled by sudden strokes of the tail fin. As the animal glides after each stroke, 

 the abdomen is folded under; coming to rest upon the bottom, the animal 

 lifts and expands the abdomen in preparation for another stroke. Crayfishes 

 respond quickly to visual stimuli; but in burrows, where little light penetrates, 

 and in their nocturnal activities generally, various receptor organs for tactile 

 and chemical stimuli must be more significant. Such receptors are generally 

 distributed over the body but are most numerous on the two pairs of anten- 

 nae. In feeding, the crayfish captures animals, such as aquatic insects and 

 fishes, by lying in wait and seizing them with its claws. It also lives as a 

 scavenger, feeding upon the bodies of animals found dead upon the bottom. 



Crayfishes are primarily aquatic animals, but in the laboratory they thrive 

 best if kept where they can crawl out of the water, and they will often remain 

 exposed in a moist atmosphere for hours. In their natural habitats, along 

 a stream at night, crayfishes are sometimes seen upon the bank near the 

 water; they occasionally make nocturnal expeditions of some length upon 

 land, possibly in search of food. 



General Structure. As in all segmented animals, the body of the crayfish 

 is composed of a series of somites. These are not all similar, however, but 

 are grouped together and modified to form definite, specialized regions of the 

 bodv. Three regions, the head, the thorax, and the abdomen, may be dis- 

 tinguished externally. The head and the thorax are fused to form the so- 

 called cephalothorax, covered dorsally and laterally by a non-segmented 

 portion of the e.xoskeleton, the carapace. The carapace terminates anteriorly 

 in a pointed rostrum, and the boundary between head and thorax is marked on 

 the carapace by a transverse cervical groove. In these anterior regions the 

 underlying segmentation of the body, obscured dorsally by the carapace, is 

 revealed ventrally by the segmental origin of a series of paired appendages. 

 In the abdomen the somites are clearly demarked both dorsally and ventrally. 



The paired eyes and two pairs of sensory appendages, antennules and 

 antennae, project laterally and anteriorly from their attachments beneath the 

 rostrum. The appendages about the mouth, which are modified to assist in 

 the capture and manipulation of food, are distinguished as oral appendages, 

 or "mouth parts" (Fig. 15.3). Of these, the mandibles and two pairs of 

 maxillae originate from segments of the head; the following three pairs of oral 

 appendages, the maxillipeds, originate from the three anteriormost segments 

 of the thorax. Behind the third maxillipeds arise the great pincer-bearing 

 appendages, the chelipeds, which constitute the first of five pairs of walking 



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