100 MANUAL OF ANIMAL BIOLOGY 



(rostrum). In the posterior portion of the body (abdomen) 

 the segmentation is clearly marked. The exoskeleton covering 

 each abdominal segment consists of a curved dorsal portion 

 (tergum) connected by a ventral, transverse bar (sternum) . The 

 ends of the tergum (pleura) project ventrally below the sternum. 

 (W. f. 65.) 



Altogether there are twenty-one segments in the body of the 

 Crayfish, of which segments i-vi constitute the head, segments 

 vii-xiv constitute the thorax, and segments xv-xxi the abdo- 

 men. Attached to every segment except the first and last is a pair 

 of jointed appendages. The first segment bears the eyes, but 

 these are not usually regarded as being homologous with the 

 appendages. Also the last segment of the body, the telson, does 

 not possess appendages, thus leaving a total of nineteen pairs of 

 appendages. (W. f. 63.) 



B. Structure of the Appendages 



The appendages of the Crayfish may be divided into two general 

 types as follows (W. fs. 63, 64) : 



(1) The biramous appendage is found in a typical form on 

 most of the abdominal segments and in a highly modified form on 

 the head segments. A typical biramous abdominal appendage 

 consists of a basal portion, the protopodite, which is composed 

 of two segments : the coxopodite, attached to the body, and the 

 basipodite. The latter bears two jointed branches : the inner 

 termed the endopodite, and the outer the exopodite. This form 

 of appendage is believed to represent the primitive and fundamental 

 type from which the modified types of appendages have been 

 derived. 



(2) The uniramous appendage is represented by the walking 

 legs. In this modified type of appendage the protopodite shows 

 the same general structure as in the biramous appendage, but the 

 exopodite is always lacking, so that the basipodite bears only one 

 branch, the endopodite. Attached to the coxopodite of certain of 

 the walking legs is a gill-bearing structure, the epipodite, which 

 aids in respiration as indicated later. 



In giving a description of the appendages of the Crayfish, we 

 shall begin with those present on the abdomen inasmuch as they 

 are more simple and are believed to represent, as noted above, the 

 fundamental type. 



