IV PORIFERA 57 



sperm cell. The gametes are developed in the mesogloea, 

 usually in the autumn, and the egg remains hidden in the 

 tissues of the parent during the first stages of its develop- 

 ment, making its way out of an osculum as an independent 

 organism only when it has developed into an oval multi- 

 cellular ciliated body ; this young form swims freely in the 

 water for several days, but it finally fixes itself to some 

 object, and for the rest of its life lives a stationary existence, 

 gradually growing into a complex sponge like its parent. 

 The Source '^^^ power possessed by sponges of extracting 

 of Silica in silica from the water in which they live, is re- 

 Sponges, markable, for the amount of this found in solution 

 in sea-water is very insignificant, being about 1^ parts of 

 silica in 100,000 of water. It is said that to form 1 oz. of 

 the spicules at least one ton of water must pass through 

 the body of the sponge. This same power of extracting the 

 silica is also exercised by certain Protozoa (the Radiolaria), 

 and by the little microscopic plant forms, the Diatoms. It 

 is interesting to note that the formation of the flints so often 

 found embedded in chalk rock, is said to be connected with 

 the remains of sponges. 

 Relation- I** i^ clear from the above account that Porif era 

 ships of resemble Coelenterates in having the two tissues, 

 the Porifera. ectoderm and endoderm, well developed, and in 

 having mesogloea separating them ; yet they are peculiar in 

 very many points, such as the presence of inhalent pores and 

 oscula, the course of the water current in the body, the posses- 

 sion of the singular " collar cells," and the characteristic sponge 

 spicules formed in the mesogloea of most of them. Hence 

 they are classed in a separate phylum. Probably they have 

 been quite independently derived from a Protozoan ancestor, 

 and are more primitive than Coelenterates, though they 

 have been described here after the latter, for their very 

 peculiar structure is then more evident and is more fully 

 appreciated. Their connection with the Protozoa becomes 

 more apparent when the order of the Choanoflagellata 

 amongst the Protozoa is studied,^ for these forms have the 

 same curious '' collar " round the base of the flagellum, which 

 is so characteristic of the flagellate cells in Sponges ; also 

 in one Protozoan form, Protero-spongia,^ a number of indi- 



' See article " Protozoa," Encyclopcedia Britannica. 



