in PHYLUM AND CLASS PORIFERA 115 



clear cells disappear at this stage, and the ectoderm cells become 

 amoeboid and lose their granular character. The opening of the 

 cup or gastrula the Uastoporc at first a wide opening, soon 

 becomes narrowed, and eventually closes up completely. A clear 

 layer containing cells the mesogloaa has now become developed 

 between the other two, and in this the first spicules become 

 developed. The embryo has meanwhile become fixed by the side 

 on which the blastopore was situated (Fig. 82, #), and sooii assumes 

 a cylindrical form (Fig. 82, 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 and its cells becoming flagellate. 



The amphiblastula type of larva is characteristic of the Calcarea, 

 and is probably universal in that sub-class, except in such primi- 

 tive forms as Ascetta. In the latter there is an oval blastula with 

 a wall composed of a single layer of flagellate cells. From the 

 posterior pole of the blastula, where the cells are more granular, 

 cells pass into the segmentation cavity, which they eventually 

 completely fill. The central mass of cells thus formed gives rise 

 to the collar-cells of the flagellate chambers, the outer layer to the 

 flattened ectoderm. 



In the Silicispongia?, on the other hand, the typical larva is a 

 solid body with a superficial layer of ciliated, 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 flattened cells lining the canals. The granular 

 cells break through the ciliated 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 

 Coelenterata 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 remains 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 being all members of existing groups. Some of the orders 

 of existing Sponges such as the Myxaspongias are incapable of 

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

 expect, to the more highly silicified Non-Calcarea and to the more 

 complex groups of the Calcarea. 



Fresh-water Sponges (Spongillidce) 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 shun 



