DEVELOPMENT. 51 



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

 bly more effectively, by the contraction and dilatation of the 

 foot. 



At this stage the larvae -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 

 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 embryos 

 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 by 

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

 posed are driven down into their interior, which 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 portion of the prosobran- 

 chiates. 



Before dismissing the subject of development, we must refer 

 briefly to the temporary larval existence through which a por- 



* Eept. Brit. Assoc., 108, 1854. 



