iii PHYLUM PORIFERA 89 



the earlier stages of its development. Eventually it becomes 

 free as a ciliated larva, which pursues a free existence for a 

 time, swimming about by the agency of the cilia, till after 

 a time it becomes fixed and develops into the adult form. 



Fresh-water sponges (Spongilhda) live in rivers, lakes, etc. 

 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 animal 

 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, mol- 

 luscs, and other invertebrates. None of the sponges are 

 true parasites. The little boring sponge, Cliona, burrows 

 in the shells of oysters and other bivalves, and even into 

 solid limestone, but for protection and not for food. But 

 the sponge frequently lives in that close association with 

 another animal or plant to which the term messmateism, or 

 commensalism is applied associations which benefit one or 

 both. Thus some species of sponge are never found grow- 

 ing 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. Cer- 

 tain cirripede 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 (Hyalonema} 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). 



