VIII 



PHYLUM MOLLUSCOIDA 



305 



The coelome 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. 290, 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 



FIG. 293. Anterior body -wall of Terebratula, to show nervous system, &c. J,n. dorsal mesen- 

 tery ; g. brain ; gf, genital folds ; n. nephridinm ; nt. nephrostome ; ces. gullet ; ov. ovary ; c. 

 oesopbageal connective ; v.sg. infra-oasophageal ganglion ; vm. ventral mesentery ; dmn, Jin, 

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



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 

 (npli) 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. 293) is a ring round the gullet pre- 

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

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



