PHYLUM AND CLASS PORIFERA 



125 



The clear cells become elongated and flagellate. The archseocytes 

 pass into the internal (segmentation) cavity and become completely 

 enclosed by the flagellate cells (stage of so-called pseudogastrula). 



The cells at the posterior end then lose their flagella and 

 become large rounded granular cells, so that after a time the 

 wall of the embryo comes to be composed in one half of the 

 flagellate cells that have remained un- 

 altered, and in the other half of the larger 

 granular cells. It is in this stage termed 

 the amphiblastula (Fig. 95, e) that the 

 larval sponge becomes free. At a later 

 stage the flagellate cells are partly over- 

 grown by the granular cells, the latter 

 eventually giving rise to the ectoderm 

 of the adult, while the former become 

 the flagellate collared cells. The larva 

 fixes itself by one side, and soon 

 assumes a cylindrical form (h, i). An 

 aperture which is developed at the free 

 end becomes the osculum, and small 

 perforations in the sides of the cylinder 

 form the inhalant apertures. As the wall 

 of the cylinder increases in thickness by 

 the growth of the mesoglcea, the radial 

 canals are formed, the endoderm extending 

 into them, its cells remaining flagellate to 

 form the choanocytes of the radial canals. 



The amphiblastula type of larva is characteristic of the Calcarea, 

 and is probably universal in that sub-class except in such primitive 

 forms as Clathrina. 



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

 body with a superficial layer of flagellate cells, and an internal mass 

 of granular cells. From the former, apparently, the collared cells 

 of the flagellate chambers are formed : from the latter the external 

 ectoderm and the other elements of the body of the Sponge. The 

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

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

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

 Ccelenterata and in fact in the rest of the Metazoa, but is readily 

 reconcilable with what takes place in Sycon and the more complex 

 Calcarea. 



Distribution and Mode of Occurrence of Sponges and 

 their Position in the Animal Series. Fossil remams of Sponges 

 have been found in various formations from those of the Cambrian 

 period onwards, the greatest abundance being found in the Chalk. 

 No extinct class or order has been detected, the fossil forms all 



FIG. 94. Median longitudiiia 

 section of the parenchymula 

 larva of Clathrina blanca. 

 p.ff.c., posterior granular cells 

 which give rise to the archseo- 

 cytes. (From the Cambridge 

 Natural History, after Min- 

 ehiii.) 



