126 ZOOLOGY sect. 



The amphiblastula tj'pe of larva is characteristic of the Calcarea, 

 md is probably universal in that sub- class except in such primi- 

 ;ive forms as Clfithrina. 



In the Silicispongia3, on the other hand, the typical larva is a solid 

 Dody with a superficial layer of ciliated cells, and an internal m'ass 

 )f granular cells. From the former, apparently, the collared cells 

 jf the flagellate chambers ai-e formed : from the latter the external 

 3ctoderm and the other elements of the body of the Sponge. The 

 granular cells break through the ciliated cells at one end and grow 

 Dver the latter as an investing layer. This is a remarkable reversal 

 Df what, as will be seen subsequently, is to be observed in the 

 Z!oelenterata— and in fact in the rest of the Metazoa, but is readily 

 reconcilable with what takes place in Sycon and the more complex 

 3alcarea. 



Distribution and Mode of Occurrence of Sponges, and 

 their Position in the Animal Series. — Fossil remains of Sponges 

 lave been found in various formations from those of the Cambrian 

 3eriod onwards, the greatest abundance being found in the Chalk. 

 N'o extinct class or order has been detected, the fossil forms all 

 Deing members of existing groups. Some of the orders of existing 

 Sponges — such as the Myxospongiaj — are incajDable of being 

 preserved as fossils, and the fossil forms belong, as we should 

 jxpect, to the more highly silicified groups and to the more 

 3omplex groups of the Calcarea. 



Fresh-water Sponges (S^Jonr/Ulidcc) 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. The 

 Oalcarea and the true horny sponges [Ucratosci) 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 Hcxadinellida and Gkoristida. 



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 

 bhe 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 

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

 jerm mcssmatcmn, or commensalism, is applied, associations which 

 aenefit one or both. Thus some species of Sponge are never found 

 growing except on the backs or legs of certain Crabs. In these 

 jases the Sponge protects the Crab and conceals it from its enemies, 

 ivhile the Sponge benefits by being carried from place to place and 

 }hus obtaining freer oxygenation. Certain Cirri pede Crustaceans 



