1 8 Dr. W. B. Carpenter on the Development of Purpura lapillus. 



ference of size not being accompanied by advance in develop- 

 ment, bat merely depending upon the amount of this 'sup- 

 plementaP yolk which the individuals have respectively gulped 

 down. For during the time in which they are engaged in 

 appropriating this additional supply of nutriment, although they 

 increase in size, yet they scarcely exhibit any other change ; so 

 that the large embryo, fig. 2 e, is not apparently more advanced 

 as regards the formation of its organs, than the small embryo, 

 fig. 1, K. So soon as this operation has been completed, how- 

 ever, and the embryo has attained its full bulk, the evolution of 

 its organs takes place very rapidly ; the ciliated lobes are much 

 more highly developed, being extended in a long sinuous margin, 

 so as almost to remind the observer of the 'wheels^ of Rotifera, 

 and furnished with very long cilia (fig. 2, b); the auditory vesicles, 

 the ten taenia, the eyes, and the foot, successively make their ap- 

 pearance ; a curious rhythmically-contractile vesicle is seen just 

 beneath the edge of the shell in the region of the neck ; a little 

 later, the heart may be seen pulsating beneath the dorsal part of the 

 shell ; and the mass of yolk- segments of which the body is made 

 up gradually shapes itself into the various organs of digestion, 

 respiration, &c., during the evolution of which (and while they 

 are as yet far from complete) the capsule thins away at its sum- 

 mit, and the embryos make their escape from it. It happens 

 not unfrequently that one of the embryos which a capsule con- 

 tains, does not acquire its supplemental yolk in the manner now 

 described, and can only proceed in its development as far as its 

 original yolk will afford it material ; and thus, at the time when 

 the other embryos have attained their full size and maturity, a 

 strange-looking creature, consisting of two large ciliated lobes 

 with scarcely the rudiment of aL body, may be seen in active mo- 

 tion among them. This may happen, indeed, not only to one 

 but to several embryos within the same capsule, especially if 

 their number should be considerable; for it sometimes appears 

 as if there were not food enough for all, so that whilst some 

 attain their full dimensions and complete development, others 

 remain of unusually small size, without being deficient in any of 

 their organs, and others again are more or less completely abor- 

 tive, — the supply of supplemental yolk which they have obtained 

 having been too small for the development of their viscera, 

 although it may have afforded what was needed for that of the 

 ciliated lobes, eyes, tentacles, auditory vesicles, and even the 

 foot, — or, on the other hand, no additional supply whatever 

 having been acquired by them, so that their development has 

 been arrested at a still earlier stage. 



Now, in defence of the foregoing explanation of the process, 

 and in reply to the criticisms of MM. Koren and Danielssen, I 



