PORIFERA. 181 



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 seaweeds 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 uncommon 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 

 excrescence." 



In the second order, the Hyppocrepia, all the members are inha- 

 bitants of fresh water; one of the most common species, and that 

 which attracted the attention of Trembley as long ago as 1741, is the 

 AlcyoneUa 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 polyp "is organi- 

 cally connected with the mass, the tube forming its tunic, from which 

 the animated body issues by a process of evolution similar to that 

 which develops the horn of a snail. When developed, the head pro- 

 jects a short way, and is crowned with a beautiful expansion of tenta- 

 cula, 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 

 form. About 1600 polyps are situated on a square inch of surface of 

 the mass, consequently the number of polyps in one specimen, which 

 weighed 17 ounces, and measured 14^ inches in circumference, 'may 

 be computed at 106,000, and the tentacula at 5,320,000 !'" 



Trembley gave an excellent and interesting account of one of the 

 family of AlcyoneUa, as far as the powers of the microscope at that time 

 allowed. " This is one of the many kinds of water animals which live 

 as it were in societies ; of which some sorts hang together in clusters, 



