xii.] THE FRESH-WATER CRAYFISH. 131 



clucible to modifications of a fundamental form, consisting 

 of a basal joint (protopodite) with three terminal divisions 

 (endopodite, exopodite, epipodite). 



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 sys- 

 tem, the muscles and the reproductive organs, being dis- 

 posed so as to be symmetrical in relation to the median 

 vertical plane of the body. 



The wide gullet leads almost vertically into the spacious 

 stomach, and both are lined by a chitinous continuation of 

 the exoskeleton. The stomach is divided by a transverse 

 constriction into a spacious cardiac, and a much smaller 

 pyloric division, from which latter the intestine passes. The 

 walls of the anterior half of the cardiac sac are thin and 

 membranous, but, in the posterior half, they become calci- 

 fied so as to give rise to a gastric skeleton of considerable 

 complexity. The chief part of this skeleton consists of a 

 median dorsal T-shaped 'cardiac' ossicle, the cross-piece 

 of which forms a transverse arch, while its long median 

 process extends backwards in the middle line. The ends of 

 the transverse arch are articulated obliquely with two small 

 ' antero-lateral ' pieces, the extremities of which again are 

 articulated with postero-lateral pieces, and these unite with 

 a cross-piece, the ' pyloric ' ossicle, which arches over the 

 roof of the pyloric division of the stomach. In this manner 

 a sort of hexagonal frame with moveable joints is formed, 

 and the median process projects backwards so far, as to end 



92 



