VITROLLUIA FERTILIS. 47 



of the trabeeuliie. A part of my material is contained in sections 

 of the wall of the mother-sponge ; the rest was removed from 

 the wall l)v means of needles nnder the dissecting microscope 

 and then prepared either in toto or laid out into sections (10- 

 20 n thick) with all the cares necessary for the microtomizing 

 of such small objects. On examining the preserved sponge-wall 

 with transmitted light, the larva present within could be recog- 

 nized by its opaqueness and by its peculiar shape ; it could thus 

 ])e isolated without much difficulty. 



The larva in the fully developed state is spindle-shaped, the 

 broadest part of the ventricosity lying not in the middle but 

 nearer to one of the pointed ends than to the other (figs. 20 and 

 21). The broader half of the body is presumably the anterior. 

 Total length of body, 275 tj. ; greatest breadth, 88 ix. Cross- 

 section of the body, circular. 



The oldest larva I have seen in Leucopsacas orthodocus and 

 which I have figured in my Contrili. III., PI. III., fig. 25, is 

 approximately spherical in shape (about 100 // dia.). If I am 

 right in considering that larva to be fully developed or at any 

 rate not far removed from that stage, it follows that Hexactinel- 

 lidan larvœ are subject to a certain variation as regards their ex- 

 ternal form. 



In the larva of the present species, a distinct epithelium, 

 4 ij. thick at the thickest part and consisting of approximately 

 cubical cells arranged in a single layer, covers the external sur- 

 face for at least the greater part of its extent. Towards both 

 narrowed ends of the body, the layer gradually grows thinner, 

 finally to become altogether unrecognizable. I hold it probable 

 that the covering layer is i]i foct wanting at the poles, leaving 



