386 THE CRAYFISH CHAP. 



differentiation of the appendages. When hatched the 

 young animal agrees in all essential respects with the 

 adult, but its proportions are very different, the cephalo- 

 thorax being nearly globular and the abdomen small. 

 For some time after hatching the young crayfishes cling 

 in great numbers to the pleopods of the mother by means 

 of the peculiarly hooked chelae of the first pair of legs. 



All the members of the phylum Arthropoda (p. 360) 

 are characterised by each typical segment of the body 

 bearing a pair of appendages divisible into podomeres ; 

 in addition to this, there is an almost universal absence 

 of cilia in the phylum, the sperms are usually non-motile, 

 the muscles are nearly always of the striped kind, and 

 the body-cavity, which does not correspond to a true 

 ccelome, is largely represented by blood-spaces in free 

 communication with the circulatory system. 



The phylum is divided into several classes, which are 

 all air-breathing except the Crustacea, the class to which 

 the crayfish belongs and which also includes the 

 Lobsters, Crabs, Shrimps, Wood-lice, Barnacles, Water- 

 fleas, &c. : in these, when special respiratory organs are 

 present, they have the form of gills. 



PRACTICAL DIRECTIONS 



CRAYFISH 



In a living specimen note the cephalothorax, abdomen, 

 jointed appendages, and exoskeleton, as well as the mode of 

 walking and swimming. Holding the animal out of water 

 between the thumb and finger, observe that bubbles are 

 continually being formed on either side of the lower part 

 of the head (respiratory movements, see B, i). Kill 

 with chloroform (p. 31), and, at the end of the day's work, 

 preserve your dissection in 70 per cent, spirit or 2-3 per 

 cent, formaline. 



