OF THE NATURE OF SPONGES. 53 



sponge are quite visible to the naked eye, and are seen dis- 

 seminated through the whole texture of the animal in the 

 winter season. They are bodies of a yellow colour, some- 

 what translucent, pear-shaped, tapering more or less at 

 their narrow end in different species; their whole outei- sin-- 

 face is covered with delicate projecting cilise ; and when 

 viewed through the microscope, in connection with the 

 parent, we see that the rapid vibration of these ciliae pro- 

 duces a distinct current in the water immediately around 

 them, flowing always fi-om their rounded free end towards 

 their tapering fixed extremity, thus assisting the small 

 granular bodies in producing the currents of the sponge, 

 during the period of their attachment to the body. They 

 separate from the canals, and are propelled through the 

 fecal orifices early in spring. None of these ova are seen 

 in the sponge in summer, though we can detect no diffei- 

 ence in the velocity of the currents at that period. For 

 some time after they are propelled from the interior of the 

 sponge, they swim about by means of the ciliae on their 

 siuface, and exhibit all those extraordinary phenomena of 

 spontaneous motion which Cavolini, nearly half a century 

 ago, discovered in the ova of the Gorgonia and IMadrepore.* 

 They at length fix themselves, like the ova alluded to, on a 

 spot favourable for their growth ; they lose entirely their 

 original form, and become a flat transparent circidar film, 

 through which horny fibres shoot ; they soon spread, and 

 assume a form somewhat similar to that of the parent." f 



* Except that of being unable to change their shape, as the Doctor 

 afterwards discovered. Edin. Phil. Joiirn. xiv. p. 1J9; and Edin. New 

 Phil. Journ. i. p. 154. 



'}• Edin. Phil. Journ. xiii. p. :18l-.'>; and Edin. New Phil. Journ. iU 

 p. 128-1;54. 



The minute description wiiicli Dr Grunt has given oi the structure oi' 



