DEVELOPMENT. 51 



but often, as in Paludina, this circulation is assisted, and pvoba- 

 bl}'^ more effectively, by the contraction and dilatation of the 

 foot. 



At this stage the larvie 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 onlj^ 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 

 finally 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 ovum from which it 

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

 eggs is evidently appropriated by the comparatively few embr3'os 

 which are thus developed at its expense. Prof. Carpenter ex- 

 amined a large quantity of capsules, in which a considerable 

 number of small, free embryos presented themselves before the 

 conglomeration of the great mass of the ova, so that he could not 

 doubt they were generated independently- of it. The embryos 

 soon attach themselves to the conglomerate yolk-mass, and bj' 

 the action of their cilia, the small segments of which it is com- 

 posed are driven down into their interior, Avhich is soon distended 

 by them. The bodies which coalesce after segmentation, Prof. 

 Carpenter regards as imperfectly fertilized ova, and they evi- 

 dently supplement the insufficient supply of nutriment contained 

 in the yolk-sack of each developing embryo. A similar con- 

 sumption of a portion of the ova takes place in Buccinum and 

 Nassa and very probably in a large i)ortion of the prosobran- 

 chiates. 



Before dismissing the subject of development, we must refer 

 briefly to the temporary larval existence through which a por- 



* Bept. Brit. Assoc, 108, 1854. 



