126 ELEMENTARY BIOLOGY. [XII. 



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 disposed 

 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 con- 

 striction 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 calcined 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 ossicle, the cross-piece of which forms a transverse 

 arch, while its long median process extends backwards in the 

 middle line, and is connected with a very strong tooth, which 

 projects into the gastric cavity in front of the aperture of 

 communication between the cardiac and pyloric divisions of 

 the stomach. The ends of the transverse arch are articulated 

 with two lateral pieces, each of which bears a similar tooth. 

 The extremities of these antero-lateral pieces again are arti- 

 culated with postero-lateral pieces, which unite with a cross- 

 piece 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 dentigerous piece 

 projects backwards so far, as to end below the posterior cross- 

 piece. It is connected with this, however, by a short ossicle 

 which ascends obliquely forwards and is articulated with the 



