614 EMBRYONIC DEVELOPMENT OF GASTEROPODS. 



which is seen in it consists in its extension into a sort of fin-like 

 membrane on either side, the edges of which are fringed with 

 long Cilia (Fig. 323, h — l, c), whose movements may be clearly 

 distinguished whilst the Embryo is still shut-up within the egg ; 

 at a very early period may also be discerned the ' Auditory Vesi- 

 cles ' (k, x) or rudimentary Organs of Hearing (§ 482), which 

 scarcely attain any higher development in these creatures during 

 the whole of life ; and from the immediate neighbourhood of these 

 is put-forth a projection, which is afterwards to be evolved into 

 the ' Foot ' or Muscular disk of the animal. While these organs 

 are making their appearance, the Shell is being formed on the 

 surface of the posterior portion, appearing first as a thin covering 

 over its hinder part, and gradually extending itself until it be- 

 comes large enough to enclose the Embryo completely, when this 

 contracts itself. The Ciliated Lobes are best seen in the Embryoes 

 of Nudibranchs, in which they are much larger than in Lymnseus ; 

 and the fact of the universal presence of a Shell in the embryoes 

 of the former group is of peculiar interest, as it is destined to 

 be cast-off very soon after they enter upon active life. These 

 Embryoes may be seen to move-about as freely as the narrowness 

 of their prison permits, for some time previous to their emersion ; 

 and when set free by the rupture of the egg-cases, they swim 

 forth with great activity by the action of their ciliated lobes, — 

 these, like the 'Wheels' of llotifera, serving also to bring food to the 

 mouth, which is at that time unprovided with the reducing appa- 

 ratus subsequently found in it. The same is true of the Embryo 

 of Lymnceus, save that its swimming movements are less active, in 

 consequence of the inferior development of the ciliated lobes ; and 

 the currents produced by these seem to have reference chiefly to 

 the provision of supplies of food, and of aerated water for respi- 

 ration. The disappearance of the Cilia has been observed by Mr. 

 Hogg to be coincident with the development of the Teeth to a 

 degree sufficient to enable the young Water-Snail to crop its Vege- 

 table food ; and he has further ascertained that if the growing 

 animal be kept in fresh water alone for some time, without vege- 

 table matter of any kind, the gastric teeth are very imperfectly 

 developed, and the cilia are still retained.* 



478. A very curious modification of the ordinary plan of Develop- 

 ment is presented in the Purpura lapillus; and it is probable 

 that something of the same kind exists also in Buccinum, as well as 

 in other Gasteropods of the same extensive Order {Pectinibran- 

 chiata). — Each of the capsules already described (§ 476) contains 

 from 500 to 600 Egg-like bodies (Fig. 324, a), imbedded in a 

 viscid gelatinous substance ; but only from 12 to 30 Embryoes 

 usually attain complete development ; and it is obvious from the 

 large comparative size which these attain (Fig. 325, b), that each 



* See "Transact, of Microsc. Soc," 2nd_£er., Vol. ii. (1854), p. 93. 



