SPONGILLA. 151 



they may be readily confounded. " These bodies," says Mr J. 

 Hogg, " are very minute, though some are less than others, and 

 are plainly visible to the naked eye ; they are white, of a some- 

 what globular, or rather, more oval shape, the lower or smaller 

 portion being opake, and the upper transparent and membran- 

 ous. Their movements in swimming wei*e no less astonishing- 

 than elegant ; ascending from the sponge at the bottom of the 

 water to the surface ; floating gently on the surface ; or travers- 

 ing the middle of the fluid, like a balloon in the air ; or sus- 

 pending themselves nearly in one spot ; or whirling round and 

 round, describing larger or smaller circles in the water ; ap- 

 proaching or avoiding each other ; but, when perfozining their 

 quicker progressions, they move along on their sides with their 

 rounder ends precedent." 



Since the surface of these seminal granules is not ciliated, 

 but naked and smooth, it seems probable that their motility is 

 dependant on the action of endosmose. The power to move 

 about continues in them no longer than is necessary for their 

 separation from the parent mass, ceasing after the lapse of at 

 most one or two days. Having become fixed, the granules first 

 lose their spherical form, and begin to spread over their site as 

 a thin transparent film. There are at first no spicula, but 

 they soon make their appearance, originating from no fixed point, 

 nor evolved in any certain order, but in rapid succession, and, 

 what is remarkable, they have the same figure and size in this 

 primordial spot as they have in the adult sponge. " The ovum," 

 according to Dr Grant, " in spreading, changes its circular 

 form for an oblong or irregular outline, but its spreading mar- 

 gins are always surrounded with a very thin homogeneous film, 

 while its granular bodies and spicula occupy chiefly the convex 

 middle part. I have observed, however, spicula quite isolated 

 make their appearance in the spreading marginal film. lilone 

 of the spicula are ever observed to shoot their points naturally 



