DEVELOPMENT OF PUKPUKA 



937 



tion. The disappearance of the cilia has been observed by Mr. Hogg 

 to be coincident with the development of the teeth to a degree suf- 

 ficient 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, without vegetable matter of any 

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

 are still retained. 1 



A very curious modification of the ordinary plan of development 

 is presented in Pnrpnra lapilliis, and it is probable that something 

 of the same kind exists also in Hucci/ittm, as well as in other Gas- 

 tropods of the same extensive order (Pectinibranchiata). Each of 

 the capsules already described contains from 500 to 600 egg-like 

 bodies (fig. 712, A) imbedded in a viscid gelatinous substance ; but 

 only from twelve to thirty embryos usually attain complete develop- 

 ment, and it is obvious, from the large comparative size which these 

 attain (fig. 713, B), that each of 

 them must include an amount of 

 substance equal to 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 Buccinuni) seems to 

 be as follows. Of those 500 or 

 600 egg-like bodies, only a small 

 part are fertile ova, the remainder 

 being unfertilised eggs, the yolk 

 material of which serves for the 



nutrition of the embryos in the FIG. 712. Early stages of 

 later stages of their intracapsular 

 life. The distinction between 

 them manifests itself at a very 

 early period, even in the first 

 segmentation ; for, while the latter 



divide into two equal hemispheres (fig. 712, B), the fertilised ova 

 divide into a larger and a smaller segment (D) ; in the cleft between 

 these are seen the minute ' directive vesicles,' which appear to be 

 always double, although from being seen 'end on,' only one may 

 be visible ; and near these is generally to be seen a clear space 

 in each segment. The difference is still more strongly marked in 

 the subsequent divisions ; for whilst the cleavage of the infertile 

 eggs goes on irregularly, so as to divide each into from fourteen to 

 twenty segments, having no definiteness of arrangement (C, E, F, G), 

 that of the fertile ova takes place in such a manner as to mark out 

 the distinction already alluded to between the ' cephalic ' and the 

 ' visceral ' portions of the mass (H), and the evolution of the 

 former into distinct organs very speedily commences. In the first 

 instance a narrow transparent border is seen around the whole 

 embryonic mass, which is broader at the cephalic portion (I) ; next, 



_ embryonic 



development of Purpiint lajjiU/ts: A, 

 egg-like spherule ; B, C, E, F, G, suc- 

 cessive stages of segmentation of yolk- 

 spherules ; D, H, I, J, K, successive 

 stages of development of early embryos. 



1 See Trans, Microsc, Soc. ser. ii. vol. ii. 1854, p. 93. 



