EEPRODUCTION OF SPONGES 563 



detached from the parent structure, and swim forth, by means 

 of the cilia, through the large orifices in which the canals termi- 

 nate. In this condition they appear as egg-shaped particles of 

 gelatinous matter, the greater part of whose surface is covered 

 with cilia ; but the smaller end, by which they were previously 

 attached, is left bare. By the vibration of these cilia they swim 

 about for some time, very much in the manner of Animalcules. 

 Their motions are equal, smooth, and gliding ; and they appear 

 to be influenced by each other's proximity. When they ap- 

 proach near to each other, as is sometimes observed, they often 

 arrest their course, and swim for a short time round each other. 

 They appear by these movements to seek a place adapted to 

 their development ; and to this they seem partly guided by the 

 influence of light, which they seek to avoid. The nature of the 

 site chosen varies with the species. Some incrust the surfaces 

 of loose stones or shells ; others stand erect upon a solid base ; 

 and others hang from the under surfaces of projecting rocks. 

 Sometimes earthy spicula may be observed in the gemmules, 

 before they have attached themselves ; and in those species which 

 possess them in the perfect form, they always appear within a 

 short time after they are fixed. No trace of internal canals, how- 

 ever, can be discovered at this period; and the formation of 

 these does not begin until after the gennnule has completely 

 changed its condition. 



1230. When it attaches itself to any surface, the gemmule 

 adheres by its small extremity, and the cilia continue their action 

 for some little time afterwards, as if to clear a proper site for the 

 extension which is immediately to take place. The gemmule 

 then spreads itself out into a flat transparent film ; and the 

 earthy or horny fibres soon begin to appear in it. It then ex- 

 hibits minute depressions on its surface, which gradually become 

 deeper, so as to form canals penetrating into it; these unite 

 with each other, and send out ramifications through the whole 

 structure as it extends; and thus the spongy mass is at last 

 fully formed. The spicula and fibres, which at first appeared 

 to be confusedly developed in the gelatinous growing membrane, 



