DEVELOPMENT. 129 



the border of the shell the body-wall is raised into a ridge, the 

 beginning of the mantle, and, as the shell grows farther forward, 

 the intestine in most prosobranchiates (not in Chiton), instead 

 of terminating posteriorly, begins to be pushed forwards, so that 

 the anus is likewise advanced with the border of the shell to the 

 right side of the bod}'. 



The phar^aix now appears as a distinct portion of the animal 

 and within it, the different parts of the lingual membrane may 

 be distinguished as the middle, inter, and lateral plates, and 

 according to Troschel's observation, the genus to which the 

 larva belongs may be already determined by the teeth. One may 

 already notice the commencement of the spiral winding of the 

 shell, and within it is contained one loop of the intestine and 

 many large cells or yolk-spheres, which become the liver. A 

 larval shell exists even in those mollusks f Nudibranchiates) 

 which have no shell in the adult state ; it is closed by means of 

 an operculum. 



As soon as the anus commences to be pushed forwards and 

 the intestine becomes a distinct canal, the body-cavity begins to 

 appear with blood in it. There is as yet nothing to be seen of 

 the heart, and the circulation of the blood is effected b}^ the 

 contraction and dilatation of the hollow foot, or often by means 

 of an elevation on the neck, consisting of a meshwork of fibres, 

 the cervical vesicle. By means of the heart, as soon as it is 

 developed, the fluid in the body-cavity, the blood, is put into 

 motion, but often, as in Paludina, this circulation is assisted, 

 and probably more effectively, by the contraction and dilatation 

 of the foot. 



At this stage the larvse mostly leave the albumen of the egg- 

 corpuscles, in which, up to this time, they have been enclosed, 

 and swim freely about by means of their velum. Finally, the 

 mantle-cavity is formed ; the mantle, heretofore simply a ridge 

 around the front of the shell, now extends itself from the body 

 as a fold and covers, with the shell, the mantle or respiratory 

 cavity, in the base of which, a contractile structure — the heart — 

 may soon be observed. The foot is developed still further, the 

 velum, the only exclusively larval organ, slowly disappears, the 

 tentacles are prolonged and in this way the swimming larva 

 slowly becomes the creeping animal — of which the various organs 

 flnall}^ attain maturity. 



Prof. W. B. Carpenter has observed* that whilst a capsule of 

 Purpura lapillus contains from 500 to 600 vitelline bodies, never- 

 theless only from 12 to 30 embryos are produced, each of these 

 having from 20 to 30 times the bulk of the ovvim from which it 

 sprang ; so that the material contained in the original mass of 



Rept. Brit. Assoc, 108, 1854. 



