386 THE CRAYFISH < HAP. 



thorax and abdomen ; and by the gradual differentiation of 

 the appendages. When hatched the young animal agrees in 

 all essential respects with the adult, but its proportions are 

 very different, the cephalothorax 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 



