564 DEVELOPMENT OF SPONGES. 



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

 these does not begin, until after the gemmule has completely 

 changed its condition. 



1145. 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 them. They 

 then exhibit minute depressions on their 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 ap- 

 peared to be confusedly developed in the gelatinous growing 

 membrane, then manifest great symmetry and method in their 

 distribution, with relation to the pores, canals, and vents. The 

 existence of this locomotive power in the gemmules of Sponges, 

 cannot of itself be regarded as an unequivocal proof of their 

 Animal nature ; since, as it has elsewhere been shown ( VEGET. 

 PHYSIOL. 424), the sporules or reproductive granules of the 

 ALG.E are equally active before fixing themselves. But in their 

 possession of cilia, which are not known to exist in the Vegetable 

 kingdom, and in the parallelism between their subsequent 

 development and that of the gemmules of the Polypifera, there 

 exist reasons in support of their Animal character, though this 

 is very feebly manifested. 



1146. The Geographical distribution of the Porifera is 

 very extensive ; indeed it may be said to be almost universal. 

 Every coast, from the Equator to the highest Polar regions, 

 furnishes some kinds of Sponge ; but they exist in much greater 

 abundance in warm latitudes than in cold, and they attain also a 

 much greater size. They are all, of course, inhabitants of the 

 water only, and if long removed from it they lose their vitality ; 

 but there are many species, which seem able to bear exposure to the 

 air between the intervals of the tide, appearing to flourish equally 

 well in deep water, or at a level which is occasionally left dry. 



