INVERTEBRATA. 



303 



obviously metamerically segmented, seven divisions being 



present. The last of these is much flattened, and is called 



the telson. The other six are alike, each showing the same 



rounded - triangular 



form in cross section 



(fig. 149). The exo- 



skeleton here eon- 



sists of two parts 



a larger dorso- 



lateral tergum and 



a smaller ventral 



sternum. A part 



of the tergum which 



projects laterally is 



called the pleiiron, Fig. 149.-TaANs^g^SEC^ON OF ABDOMEN 



and a small portion (After Hux i ey .) 



that forms part of 



the ventral surface is the epimeron. Between the latter 

 and the sternum the appendages or limbs are attached. 

 These are covered by a cuticle continuous with the general 

 exoskeleton, and jointed as that is. There is one pair of 

 these jointed appendages to each of the six abdominal 

 somites. As the telson has no appendages, it is not usually 

 considered to be a somite, but rather an unsegmented 

 terminal portion of the body (just as the urostyle is the 

 terminal unsegmented portion of the frog's vertebral 

 column). 



4. Cephalo-thorax. The fact that thirteen pairs of 

 more or less similar jointed appendages are attached to the 

 cephalo-thorax is regarded as an indication that in that 

 region there are really thirteen somites fused together ; the 

 crayfish thus being a metameric animal in whose anterior 

 region there has been a coalescence of somites masking the 

 metamerism, much as is the case with the head of Craniate 

 Vertebrates. 



5. Appendages. Although the nineteen pairs of ap 

 pendages show great variety of form, because they aro 

 adapted to a variety of functions, they can all be described by 



