THE IMPENETRABLE SEA 



little gemmule, or ''bud", is carried out by the emerging 

 water and swirled about until it at last fastens itself to a 

 piece of rock or weed. This is but one of the sponge's 

 methods of reproduction : by the breaking away of com- 

 plex buds, asexually, each one protected by a spicule- 

 sheath and capable of developing into a complete struc- 

 ture. 



But most sponges reproduce sexually by the union of 

 spermatozoon and ovum, resulting in the development 

 of a free-swimming larva (planula) — this method serving 

 not only to reproduce the species but to distribute it. 



The pores, with which the surfaces of the sponge is 

 furnished in abundance, are ordinarily of two kinds — the 

 large ones (oscula) which are few in number, and are 

 often guarded by special protective devices, such as 

 circles of spicules or muscles capable of contracting the 

 orifice ; and the far more numerous inhalant pores every- 

 where perforating the wall-surfaces of the canals. The 

 complicated "mechanism" of the sponge is still a mystery 

 — how it is able to select nutritive substances and reject 

 others ; how it is able to create and cast off its gemmules ; 

 how it digests its food; how it controls its continually- 

 waving flagella, so vital to the circulation of water 

 through its canals: everything about the sponge is a 

 baffling problem, challenging man's investigation. 



In the higher flint-building sponges the structure is 

 exquisite, for the spicules become complicated in the 

 extreme and are bound together with fine, transparent 

 flint threads which create a pattern resembling that of a 

 valuable piece of lace. The sponge known as Venus's- 

 basket is an example of this fine weaving. It seems 

 incredible that it has not been woven by an artist in lace- 

 making. It has all the beauty and transparency of finely- 

 spun glass, or crochet- work created after years of patient 

 labour by an expert in such a handicraft. 



Like the Portuguese man-of-war, the sponge is a com- 

 munity structure, in which each tiny individual does its 



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