STRUCTURE OF THE LARVA OF SPONGILLA LACUSTRIS. 421 



individual cells with vesicular uuclei surrounded by groups of 

 flagellated cells. When the plasmodial aggregations break up, 

 the cells with vesicular nuclei pass out of the group of cells 

 and plaster themselves to the outer surface of the flagellated 

 chambers subsequently formed, in the same way as the pore 

 cells of the Ascons pass out of the gastral cavity to tlie surface 

 when the sponge expands. From the physiological point of 

 view the correspondence between these two classes of cells 

 seems pretty complete. Of course it is quite needless to say 

 that the cells with vesicular nuclei do not all become porocytes, 

 for many of them retain their amoeboid character as well as 

 their blastomeric properties, and serve later on for the building 

 up of the gemmule. This discussion regarding the physiolo- 

 gical correspondence of these cells strengthens the argument in 

 favour of their morphological homology ; in other words, in 

 favour of the view that some of the cells with vesicular nuclei 

 become porocytes in Spongilla. 



The Subdermal Cavities. — These spaces are situated 

 below the dermal membrane, and communicate on the one 

 hand with the exterior by way of the inhalant ostia, and on 

 the other with the flagellated chambers b}^ way of the inhalant 

 canals and prosopyles. The cavities in question have a super- 

 ficial position, and they and their ostia are arranged on the 

 slanting sides of the young sponge, while the osculum takes up 

 a more or less central position. Deeper down, and nearer the 

 centre, the exhalant canal system and gastral cavity are found. 



The spaces which have been figured in such profusion in the 

 region between the marginal membrane and the body of the 

 young sponge are by no means all of them true ostia opening 

 into the subdermal cavities. Many of them are merely the 

 nutritive vacuoles found in the cells which have wandered into 

 the space between the two layers of the marginal membrane. 

 The vacuoles in question in many cases present the exact 

 appearance of an intra-cellular opening, the nuclei of the cells 

 lying close to them, but on focussing the microscope more care- 

 fully the cells of the flattened epithelium can be seen overlying 

 them. However, this does not do away with the presence of 



