MATERIALS FOR A MONOGRAPH OF THE ASCONS. 515 



due to the variations i)i the characters of the dermal epithelium, 

 which have already been discussed above. In none of the 

 species examined by me are the cells of the flat epithelium 

 and the actinoblasts so granular as in coriacea (compare PI. 

 39, figs. 17, 19, 20, and PI. 41, fig. 40, with PI. 38, figs. 4—7). 

 In consequence the granules, being fewer, are absorbed more 

 quickly, and the cells rapidly assume the clear, finely granu- 

 lated appearance characteristic of the older spicule cells 

 (PI. 39, figs. 18—21 ; PI. 40, fig. 33 ; and PI. 41, fig. 38). We 

 have noticed already the difference in the period at which the 

 apical cells disappear. In coriacea the apical cells have not, 

 as a rule, absorbed all their granules before their flight, but in 

 contorta and the undetermined species they become perfectly 

 clear. The spicule formation in cerebrum and reticulum 

 does not call for any special remark, and beyond satisfying 

 myself that it follows in these species the typical plan I have 

 not studied it in great detail. It is indeed by no means an easy 

 matter to follow the details closely in thick-walled species. 

 The great mass of refractile spicules crossing each other in all 

 directions makes it very difficult to follow minute details. 

 For this reason I have not verified the scheme of development 

 for the spicules of the adult Ascandra falcata, but have done 

 so for the larvse, as I shall proceed to describe. 



The development of the spicules is not difficult to follow in 

 larvae which have fixed themselves on cover-slips. Clathrina 

 blanca is rather an unsuitable species for this purpose, on 

 account of the compactness and consequent opacity of the 

 embryos, but cerebrum, reticulum, and falcata, on the 

 other hand, are favorable objects. The fixed embryos should 

 be preserved and stained with osmic acid and picrocarraiue, 

 and mounted either in glycerine or Canada balsam. The 

 first spicules appear on the second day, i. e. about twenty-four 

 hours after fixation; but new ones are continually being formed 

 during subsequent development and growth. The embryos 

 consist of a superficial layer of cubical or flattened dermal 

 epithelium, enclosing the gastral layer, which is represented 

 at first by a compact mass of cells, in which a cavity appears. 



