MARINE SPONGES. 307 



where they grow so well that they form masses as 

 big as a man's head. But some of them are not 

 attractive-looking creatures ; and, indeed, few people 

 would imagine them to be animals at all. The 

 spongy object which we know as sponge, is only 

 the inner skeleton ; just as coral is of another class 

 of marine animals when alive. This substance is 

 covered all over with a transparent gelatinous flesh, 

 called sarcode, which lines every pore and every aper- 

 ture. In and out of these currents of sea water per- 

 petually flow, induced by the action of the eyelash-like 

 processes (cilia) with which the surface of the flesh 

 lining such hollows is covered. Among them we have 

 the " crumb-of-bread " sponge (Halichondria panicea, 

 Fig. 238), the Chalina oculata, Fig. 239 (largest of our 

 British sponges), the Leucosalenia, Tetkea, Grantia, 

 Hymeniacidon, Cliona the latter is really a boring 

 sponge, and burrows the holes we may see covering the 

 surfaces of old oyster-shells. Some of these sponges 

 are of a brilliant yellow or crimson colour, and mat 

 the surfaces of rockwork, &c., having been introduced 

 into the sea water as " gemmules," and developed by 

 circumstances into their present attractive and useful 

 conditions. 



No doubt the marine aquarium might be converted 

 into a nursery for microscopic objects as well as the 

 fresh water. There is no end to the variety of the 

 lower forms of life inhabiting the sea. They are 

 the oldest of all the animal kingdom, having been in 



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