MOLLUSCOUS ANIMALS GENERALLY. 



187 



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

 and the cilia are still retained. 1 



583. A very curious modification of the ordinary plan of development 

 is presented in the Purpura lapillus; and it is probable that something 

 of the same kind exists also in Buctinwn, as well as in other Gasteropods 

 of the same extensive Order (Pectinilranchiata). Each of the capsules 

 already described ( 581) contains from 500 to 600 egg-like bodies (Fig. 

 404, A), imbedded in a viscid gelatinous substance; but only from 12 to 

 30 embroyes usually attain complete development; and it is obvious from 

 the large comparative size which these attain (Fig. 405, B), that each of 

 them must include an amount of substance equal to that of a great num- 

 ber of the bodies originally found within the capsule. The explanation 

 of this fact (long since noticed by Dr. J. E. Gray, in regard to Buccinum) 

 seems to be as follows: Of those 500 or 600 egg-like bodies, only a small 

 part are fertile ova, the remainder being unfertilized eggs, the yolk- 



FlG.404. 



FIG. 405. 



Early stages of Embryonic Development 

 of Purpura lapillus: A, egg-like spherule; 

 B, c, E, P, G, successive stages of segmenta- 

 tion of yolk-spherules; D, H, i, j, K, succes- 

 sive stages of development of early em- 

 bryoes. 



Later stages of embryonic Development of Pur- 

 pura lapillus: A, conglomerate mass of vitelline 

 segments, to which were attached the embryoes, a, 

 6, c, d, e: B, full-size embryo, in more advanced 

 stage of development. 



material of which serves for the nutrition of the embryoes in the later 

 stages of their intra-capsular life. The distinction between them mani- 

 fests itself at a very early period, even in the first segmentation; for 

 while the latter divide into two equal hemispheres (Fig. 404, B), the 

 fertilized 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 or even triple, 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 14 to 20 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 



1 See " Transact, of Microsc. Soc.," 2d Ser., Vol. ii. (1854), p. 



