420 RICHARD EVANS. 



auces represeuted iu fig. 33 is that the pore is iutra-cellular 

 passing through the large cell, the vesicular nucleus of which 

 is drawn in fig. 33^. If some of these cells do become poro- 

 cytes we would expect the nucleus to become granular. In 

 fig. 24 b, which represents a surface view drawn at different 

 levels, there are two extremely large nuclei situated deep in 

 the tissues of the individual among the small cells which are 

 evidently becoming grouped to form chambers. The cells to 

 which these two nuclei belong seem to lie more or less loosely 

 on the cells of the chamber, and it is quite possible that they 

 are cells similar to that containing the large vesicular nucleus 

 drawn at the top of fig. 33 c?^ in which the nuclei have lost 

 their vesicular character. The porocytes in the Ascons are 

 described by Mr. Minchin (11) as large cells, situated near the 

 surface when the sponge is fully expanded, which in the con- 

 tracted condition of the sponge push their way inwards between 

 the collar-cells, and ultimately come to lie inside the gastral 

 layer as a granular axis to the Ascou tube. The granules, 

 according to Topsent (14), consist of reserve food material. If 

 so, the cells probably collect it when the sponge is expanded, 

 and distribute it during the contracted condition to the other 

 cells. The passing in of the porocytes to the interior would 

 seem, therefore, to have a physiological meaning. In Ascons, 

 contraction or expansion may happen at any time in the history 

 of the sponge. Let us now turn to the pupa of Spongilla. Here 

 the large cells with vesicular nuclei, and containing food 

 material in the larva at the time of fixation, would correspond 

 to the porocytes of the Ascons in the expanded condition of the 

 sponge. During metamorphosis, however, they become sur- 

 rounded by the flagellated cells, that is by cells which are po- 

 tentially collar-cells — a condition of things found in the Ascons 

 during the contracted stage, with the difference, iu the two 

 cases, that in Ascons the gastral layer is continuous, while in 

 Spongilla it is broken up into groups of cells. Hence during 

 the contracted stage iu the Ascons we find a continuous axis 

 surrounded by an uninterrupted gastral layer; but iu Spon- 

 gilla, during the stage with plasmodial aggregations, we find the 



