90 MANUAL OF ZOOLOGY SECT, in 



Fresh-water Sponges {Spongillida) occur in rivers, canals 

 and lakes in all the great divisions of the earth's surface. 

 Marine Sponges occur in all seas, and at all depths, from 

 the shore between tide-marks to the deepest abysses of the 

 ocean. 



Sponges do not appear to be edible by fishes or even 

 the higher crustaceans or molluscs. Countless lower ani- 

 mal forms, however, burrow in their substance, if not for 

 food, at least for shelter, and the interior of a Sponge is 

 frequently found to be teeming with small Crustaceans, 

 Annelids, Molluscs, and other invertebrates. None of the 

 Sponges are true parasites. The little Boring Sponge, 

 Cliona, burrows in the shells of Oysters and other Bivalves, 

 but for protection and not for food. But the Sponge fre- 

 quently lives in that close association with another animal 

 or plant to which the term messmatetsm, or commensalism 

 is applied associations which benefit one or both. Thus 

 some species of Sponge are never found growing except on 

 the backs or legs of certain Crabs. In these cases the 

 Sponge protects the Crab and conceals it from its enemies, 

 while the Sponge benefits by being carried from place to 

 place, and thus obtaining freer oxygenation. Certain Cirri- 

 pede Crustaceans (members of the order to which the 

 Barnacles and Acorn-shells belong) are invariably found 

 embedded in certain species of Sponge. Frequently a 

 Sponge and a Zoophyte grow in intimate association, so 

 that they seem almost to form one structure. Thus the 

 Glass-rope Sponge (Hyalonemd) is always found associated 

 with a Zoophyte (Pafythoa), and there are many other in- 

 stances. Sponges often also grow in very close association 

 with certain low forms of plants (Alga). 



