m PHYLUM AND CLASS PORIFERA 127 



between tide-marks to the deepest abysses of the ocean. The 

 Calcarea and the true horny sponges (Ceratosa) are most abundant 

 in shallow water, and have not been found below 450 fathoms. 

 The Sponges found at the greatest depths are members of the 

 groups Hexactinellida and Choristida. 



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, 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 a Sponge frequently lives 

 in that close association with another animal or plant to which the 

 term messmaleism, 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 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 

 (Palythoa), and there are many other instances. Sponges often also 

 grow in very close association with certain low forms of plants 

 (Algae). 



The position of the Porifera in the animal series is unquestion- 

 ably among the Metazoa. The view that they are compound 

 Protozoa is now no longer maintained since the significance of 

 the facts of their development has been fully recognised. A 

 Sponge is to be regarded as a colony of Protozoa only in the sense 

 in which the same may be said of one of the higher animals. It 

 consists of a complex of cells, some of which have a consider- 

 able degree of independence, and some of which have a close 

 resemblance to certain Protozoa ; but the same is true of one of 

 the higher animals, the difference being one of degree and not 

 of kind. Like the rest of the Metazoa, the Sponge develops from 

 the oosperm by a process of yolk-division. 



But the Porifera are perhaps somewhat nearer the Protozoa 

 than are any of the other types of Metazoa ; and among the 

 Protozoa they appear to approach nearest to certain colonial 

 Flagellata. The genus Proterospongia (Fig. 59), already referred 

 to (p. 80), appears to be the member of the latter group which of 

 all known forms most closely resembles a sponge. Proterospongia 

 consists of a colony of collared Flagellates (Choanoflagellata) 



