MORPHOLOGY AND SYSTEMATIC POSITION OF SPONGIDA. 105 



the border of the clear cells. In the centre of the embryo is a 

 segmentation cavity (cs) which lies between the granular and the 

 clear cells, but is mainly bounded by the vaulted inner surface of 

 the latter. This stage is known as the araphiblastula stage. After 

 the larva has for some time enjoyed a free existence, a remarkable 

 series of changes takes place, which result in the invagination of 

 the half of it formed of the clear cells, and form a prelude to the 

 permanent attachment of the larva. The entire process of invagi- 

 nation is completed in about half an hour. The whole embryo 

 first becomes flattened, but especially the ciliated half which 

 gradually becomes less prominent (fig. 1, b), and still later the 

 cells composing it undergo a true process of invagination. As a 

 result of this invagination the segmentation cavity is obliterated 

 and the larva assumes a compressed plano-convex form with a 

 central gastrula cavity, and a blastopore in the middle of the 

 flattened surface. The two layers of the gastrula may now be 

 spoken of as ectoderm and entoderm. The blastopore becomes 

 gradually narrowed by the growth over it of the outer row of 

 granular cells. When it has become very small the attachment 

 of the larva takes place by the flat surface where the blastopore 

 is situated. It is effected by protoplasmic processes of the outer 

 ring of ectoderm cells, which, together with the other ectoderm 

 cells, now become amojboid. At the same time they become 

 clearer and permit a view of the interior of the gastrula. Be- 

 tween the ectoderm cells and the entoderm cells which line the 

 gastrula cavity there arises a hyaline structureless layer, wliich 

 is more closely attached to the ectoderm than to the entoderm, 

 and is probably derived from the former. A view of the gastrula 

 stage after the larva has become fixed is given in fig. 2. 



Fig. 2. 



Fixed Gastrula stage of Sycandra rmhanus (copied from Schuize). 

 The figure shows the amoeboid ectoderm cells {ec) derived from tiie granular 

 cells of the earlier stage, and the columnar entoderm cells, lining the 

 gastrula cavity, derived from the ciliated cells of the earlier stage. The 

 larva is fixed by the amoeboid cells on the side on which the blastopore 

 is situated. 



