336 



ZOOLOGY 



SECT. 



pair of (Uraricators (d. m, d. m') arise from the ventral valves, and 

 are inserted into the cardinal process, which they depress : as this 

 process is situated posteriorly to the hinge-line, its depression 

 raises the rest of the dorsal valve, and so opens the shell. Two 

 pairs of muscles arising, one from the ventral, the other from the 

 dorsal valve, and inserted into the peduncle, are called adjustors 

 (aj.m): the peduncle being fixed, they serve to alter or adjust 

 the position of the animal as a whole by turning it in various 

 directions. 



The ccelome is a spacious cavity more or less encroached upon 

 by the muscles and other organs, and traversed by sheets and 



53 



I-'K.. LV.'.'. Anterior body-wall of Terebratula, to show nervous system, &c.: dm. dorsal mesen- 

 tery : .'/. brain ; <jt. genital folds ; n. nephridium ; nt. nephrostome ; o:s. gullet ; or. ovary ; sc. 

 oesophageal connective ; usg. infra-ossophageal ganglion ; cm. ventral mesentery ; ii/nn, kn, ian, 

 fiii/. nerves. (From Lang's Comparatiit Anatomy.") 



bands of membrane which connect the enteric canal with the 

 body- wall, and thus act as mesenteries. The coelome is continued 

 into each of the mantle-lobes in the form of four canals or pallial 

 sinuses (Fig. 266, pi. s), the two outer of which are extensively 

 branched. 



Blood System. Attached to the posterior region of the 

 stomach is a small, almost globular sac (A), which has been proved 

 to be contractile, and is to be considered as a heart. Vessels have 

 been traced from it to various parts of the body, but the relations 

 of the whole circulatory system and the course of the circulation 

 are very imperfectly known. 



