181 



preserved quite fresh from the sea on board the fishing boat. The re- 

 sults so obtained were then verified and corrected by sections and by 

 isolating the cell elements. 



1) The Ectoderm. This layer, the contractile layer of the 

 sponge , consists in the expanded state of flattened, nonciliated cells, 

 each with a large spherical or sligthly ovoid nucleus containing an 

 evenly distributed chromatin network. In surface views the nuclei 

 appear at varying distances from one another , each surrounded by a 

 number of granules, and without cell outlines in ordinary prepara- 

 tions. In sections prepared by many methods the jelly is seen to 

 have a sharply defined limit , appearing as a thick line , immediately 

 under the ectoderm. The nuclei of the ectoderm are close to this 

 limiting membrane and are covered externally by a thin layer of 

 granular protoplasm. When the cells contract, the nucleus remains 

 in proximity to the limiting membrane , but the protoplasm external 

 to the nucleus thickens, and the protoplasm on each side of the nucleus 

 raises itself up from the membrane. As this process continues the 

 whole cell assumes a mushroom form, with a stalk containing the nu- 

 cleus and an expanded portion continuous with the similar portion of 

 the next cell. The accompanying diagram (Fig. 1) illustrates this pro- 



cess, showing the passage from an expanded cell on the left to a very 

 contracted one on the right. All stages of contraction can easily be 

 found in sections of a sponge preserved fresh , and if sections of an 

 open and a closed osculum ^ be compared, the external ectoderm will 

 be found in the first case to consist of flat cells, in the latter of mush- 

 room cells. There are two places, however, where the contracted 

 ectoderm does not assume this appearance , but the cells become 

 simply more rounded. The first of these places is in the muscular 

 sphincter of the osculum, the second is the ectoderm on the inside of 

 the oscular margin. The first case may be explained by the two layers 

 of the ectoderm composing the sphincter being in immediate contact, 

 and not separated by any jelly; the second is perhaps due to the cells 

 being passive and not active in the contraction. 



The mushroom like cells were first seen and figured byMetsch- 

 nikoff (1) in a y)Clistolynilius'i form of Ascetta Manca. Bidder (2) sta- 



• I have described these oscula and their sphincters in a paper shortly to appear 

 in the Quarterly .Tournai of Microscopic Science. 



