PURPURA. 523 



on the surface of tlie posterior portion, appearing first as a thin 

 covering over its hinder part, and gradually extending itself 

 until it becomes large enough to enclose the embryo com- 

 pletely, 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 Rotifera, 

 serving also to bring food to the mouth, which is at that time 

 unprovided with the reducing apparatus subsequently found in 

 it. The same is true of the embryo of Lymnseus, save that its 

 swimming movements are less active, in consequence of the in- 

 ferior development of the ciliated lobes ; and the currents pro- 

 duced by these seem to have reference chiefly to the provision of 

 supplies of food, and of aerated water for respiration. 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 vegetable food ; and 

 he has further ascertained, that if the growing animal be kept 

 in fresh water alone for some time, with vegetable matter of any 

 kind, the gastric teeth are very imperfectly developed, and the 

 cilia are still retained. 1 



351. A very curious modification of the ordinary plan of deve- 

 lopment, is presented in the Pur- 

 pura lapillus ; and it is probable FIG. 271. 



that something of the same kind o B 



exists also in Buccinum, as also in 

 other Gasteropods of the same ex- 

 tensive order (Pectinibranclnata). 

 Each of the capsules already de- 

 scribed ( 349) contains from 500 

 to 600 egg-like bodies (Fig. 271, 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 Early stages of embryonic development of 

 Which these attain (Fisr. 272, B), Purpura lapiUus: A, egg-like spherule; B,C, 



that each of them must include #3Stt&S 



an amount Of Substance equal tO of development of early embryoes. 



that of a great number of the 



bodies originally found within the capsule. The explanation of 



this fact (long since noticed by Dr. J. E. Gray in regard to Bucci- 



1 "Transactions of Microscopical Society," 2d Ser. vol. ii, p. 93. 



