CHAPTER XLII 



CRAYFISH 

 A TYPE OF THE PHYLUM ARTHROPODA 



The crayfish is a fresh-water form abundant throughout this country 

 and represented by many species. Some of these prefer quiet waters 

 and others running streams. There are also cave dwellers among them. 

 Crayfishes usually spend the day in hiding under rocks or logs or other 

 objects at the bottom of the pond or stream in which they live, though 

 they may be tempted to come out by food which comes near their place 

 of concealment. At night they become active, leaving their holes and 

 wandering freely in search of food. They are often captured in large 

 numbers in traps and may easily be caught by hand on a hook baited 

 with a piece of meat. The meat is grasped by the pincers and held with 

 such tenacity that a quick jerk will bring the animal out of the water. 

 In books they are called crayfishes, but the names more commonly 

 applied in this country are crawfishes or crawdads. 



292. External Characteristics. — Crayfishes are metameric, the met- 

 ameres being grouped into two regions, the cephalothorax and the abdomen 

 (Fig. 161). The surface of the body is covered with an exoskeleton 

 composed of chitin mingled with lime salts. On the dorsal surface of 

 the cephalothorax the skeleton forms a continuous shell known as the 

 cai'apace. A transverse cervical groove marks the division between parts 

 corresponding to the head and thorax. A median forward extension of 

 the carapace beyond the eyes is called the rostrum. 



The cephalothorax includes 13 metameres, 5 representing the head 

 and 8 the thorax, to each of which characteristic appendages are attached 

 ventrolaterally. The first metamere bears a pair of antennules, and the 

 second a pair of very long, many-jointed antennae, or feelers. On the 

 third metamere is a pair of mandibles, and on the fourth and fifth met- 

 ameres are two pairs of maxillae. A portion of the second maxilla is 

 modified to form a scooplike plate known as the scaphognathite, or bailer. 



Of the thoracic metameres, the sixth to the eighth bear maxillipeds, 

 and the ninth to the thirteenth, walking legs. In the first three pairs of 

 legs the segment next to the last is prolonged so that its tip is even with 

 that of the last, and the two together form a pincer. In the first pair this 

 pincer is very powerful, is called a chela, and the leg, a cheliped. 



Of the six abdominal metameres, the first five bear appendages 

 which are quite typical, except that the first two pairs are somewhat 



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