II.] THE CRAYFISH AND LOBSTER. 177 



sidered as at least a twentyfold repetition of the ring-like 

 somite with its pair of appendages, which is seen in its 

 simplest form in one of the abdominal somites. Moreover, 

 notwithstanding the great variety of functions allotted to the 

 various appendages, the study of the details of their struc- 

 ture (see Laboratory work) will shew that they are all re- 

 ducible to modifications of a fundamental form, consisting 

 of a basal-portion (protopodite) with two terminal divisions 

 (endopodite and exopodite). A third division or epipodite is 

 superadded to the appendages concerned in respiration. 



Of the twenty pairs of appendages, the three anterior 

 are concerned with sensation and the six posterior with 

 swimming; the mandibles, which bound the mouth, are 

 most efficient in mastication. Of the ten pairs which re^ 

 main, five are modified to form foot-jaws, the others being 

 functional for walking or climbing ; three of the latter . 

 however, in that they are chelate and often used to capture 

 prey or to tear up food material, combine the functions of 

 locomotor feet and foot-jaws. 



Each of the larger appendages is composed of a number 

 of segments, each movable upon its fellow in a single 

 plane ; the various segments are so articulated that the 

 limb, as a whole, is capable of considerable rotation. 



As has been already said, the Lobster and Crayfish are 

 bilaterally symmetrical ; that is to say, a median vertical 

 plane passing through the mouth and anus divides them into 

 two similar halves. This symmetry is exhibited not merely 

 by the exterior of the body and the correspondence of the 

 paired limbs, but extends to the internal organs ; the alimen- 

 tary canal and its appendages, the heart, the nervous system, 

 the muscles and the reproductive organs, being disposed so 

 as to be symmetrical in relation to the median vertical plane 

 of the body. 



M. 12 



