358 



ZOOLOGY 



SECT. 



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 

 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. 296, pi. si), the two outer of which are extensively 

 branched. 



Blood-system. Attached to the posterior region of the 

 stomach is a small, almost globular sac (h), 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. 



The excretory 

 organs consist of 

 a pair of very 

 large nephridia 

 (nph) lying one on 

 each side of the 

 intestine. Each 

 is funnel-shaped, 

 having a wide 

 inner opening or 

 nephrostome, with 

 plaited walls, 

 opening into the 

 coelome, and a 

 narrow, curved, 

 outer portion which opens into the mantle-cavity not far from the 

 mouth. As in many cases which have already come under our 

 notice, the nephridia act also as gonoducts. 



The nervous system (Fig. 299) is a ring round the gullet pre- 

 senting supra- (g) and infra- (usg) oesophageal swellings or ganglia, 

 of which the infra-cesophageal is the larger. Nerves are given off 

 to the mantle, lophophore, &c. No special sense-organs are known. 

 Reproductive Organs. The sexes are separate. There are two 

 pairs of gonads (Fig. 296, gori), one dorsal and one ventral, in the 

 form of irregular organs sending off branches into the pallial sinuses. 



FIG. 299. Anterior body-wall of Terebratula, to show nervous 

 system, &c. dm. dorsal mesentery ; g. brain ; gf. genital 

 folds ; n. nephridium ; nt. nephrostome ; ces. gullet ; ov. 

 ovary ; sc. oasophageal connective ; usg. infra-cesophageal 

 ganglion : vm. ventral mesentery ; dmn, Tin, ian, san. nerves. 

 (From Lang's Comparative Anatomy, after van Bemmelen.) 



