SPONGES. 279 



its surface and highest orifice with water. On strewing 

 some powdered chalk on the surface of the water, the 

 currents were visible to a great distance ; and on placing 

 some pieces of cork or of dry paper over the apertures, I 

 could perceive them moving, by the force of the currents, 

 at the distance of ten feet from the table on which the 

 specimen rested." 



Sponges grow attached to almost every thing which 

 may serve them as a point of support, whether fixed or 

 floating; some cover rocks, shells, and other submarine 

 objects, with a close spongy incrustation; whilst others 

 shoot up a branched stem into the water ; and others again 

 hang freely from the sea- weeds floating in the ocean. 

 Sometimes they select very unexpected objects on which 

 to take up their abode. Thus, in one case recorded by 

 Dr. Johnston in his Natural History of British Sponges, a 

 specimen of the Halichondria oculata, a sponge not un- 

 common on some parts of the British coasts, was found 

 growing from the back of a small live crab, " a burden," 

 says the learned Doctor, " apparently as disproportionate 

 as was that of Atlas, and yet the creature has been 

 seemingly little inconvenienced with its arboreous ex- 

 crescence." 



In the next order, Hyppocrepia, all the members are 

 inhabitants of fresh water; one of the most common spe- 

 cies, and that which attracted the attention of Trembley 

 as long ago as 1741, is the Alcyonella stagnorum. It- 

 occurs in great abundance, attached to the leaves of aquatic 

 plants, on floating logs of timber, in the West India Docks. 

 When first taken out of the water it is of a lobulated 

 form and brown colour ; the polypidom is soft and elastic, 

 and feels very much like a sponge ; but, as Mr. Teale 

 observes, this polype " is organically connected with the 

 mass, the tube forming its tunic, from which the animated 

 body issues by a process of evolution similar to that which 

 developes the horn of a snail. When developed, the head 

 projects a short way, and is crowned with a beautiful ex- 

 pansion of tentacula, about fifty in number, arranged in 

 an unbroken circle, which is, however, depressed into a 

 deep concavity on one of its sides, so as to produce the 

 appearance of a double row of tentacula, in a horse-shoe 



