PUKPURA. 



525 



Fig. 272. 



evolution of its organs takes place very rapidly; the ciliated lobes 

 are much more highly de- 

 veloped, being extended 

 in a long sinuous margin, 

 so as almost to remind the 

 observer of the "wheels" 

 ofEotifera (§ 277), and 

 being furnished with veiy 

 long ciUa (Fig. 272, b) ; 

 the auditory vesicles, the 



Later stages of embryonic development of Purpura 

 lapiltus ; — A, conglomerate mass of vilelUne segments, to 

 which were attactied the embryoes, a, &, c, d, e :— B, full- 

 sized embryo, in more advanced stage of development. 



tentacula, the eyes, and 

 the foot, successively 

 make their appearance ; a 

 curious rhythmically con- 

 tractile vesicle is seen, just 

 beneath the edge of the 

 shell in the region of the 

 neck, which may perhaps, 

 serve as a temporary- 

 heart; a little later, the 

 real heart may be seen 

 pulsating beneath the dor- 

 sal 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 summit, and the embryoes make their escape 

 from it. 



352. It happens not unfrequently, that one of the embryoes 

 which a capsule contains, 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 emlDryoes have attained 

 their full size and maturity, a strange-looking creature, consist- 

 ing of two large ciliated lobes with scarcely the rudiment of a 

 body, may be seen in active motion among them. This may 

 happen, indeed, not only to one but to several embryoes within 

 the same capsule, especially if their number should be consider- 

 able ; for it sometimes appears as if there were not food enough 

 for all, so that whilst some attain their full dimensions and com- 

 plete development, others remain of unusually small size, with- 

 out being deficient in any of their organs, and others again are 

 more or less completely abortive, — ^the supply of supplemental 

 yolk which they have obtained having been too small for the 

 development of their viscera, although it may have afibrded 

 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. 



