THE CRAYFISH 



323 



appendages of the head are six pairs of oral appendages, inchiding 

 the mandibles, which He on either side of the mouth. Posterior 

 to these oral appendages are the great claws, or chelce, which are 

 followed by four pairs of walking legs, or pereiopods. The abdo- 

 men bears the swimmerets, which are small and delicate, except 

 the most posterior pair, or uropods, which are modified as strong 

 paddle-like structures. The uropods, together with the telson, or 

 terminal portion of the bod}^ constitute the tail-fin which is no less 

 important than the pereiopods in locomotion. The anus is located 

 ventrally on the telson. 



In the special account of the appendages given on p. 325 it will 

 be seen that there are five pairs of cephalic appendages, eight pairs 

 of thoracic appendages, and six pairs of abdominal appendages. 



Fig. 155. — Crayfish viewed laterally in the attitude assumed when the 



swimmerets are being waved gently in aeration of the eggs which are attached 



to these appendages. (After Andrews.) 



Since there is a pair of appendages for each somite of the crayfish's 

 body, there are six somites in the abdomen, in addition to the 

 telson and eight somites in the thorax. In the head region there 

 are five somites plus the part bearing the eyes, which are not true 

 appendages. Thus the body has nineteen somites between the 

 telson at the posterior end and the most anterior part of the head 

 region. The structure of the internal organs, particularly the 

 nervous and circulatory systems, and the development bears out 

 this interpretation. The crayfish has, therefore, a greater degree 

 of antero-posterior differentiation than the earthworm, although its 

 metameric organization is clearly in evidence. Another feature 

 of the external organization is the strict bilateral symmetry that 

 is everywhere apparent, except in the slight difference between 

 the chelae of the two sides. The abdomen is broader in the female, 



