STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 225 



ooblasts^ attached to the rays of the large triradiate spicules 

 in the dermal cortex. Three of these cells are shown in figs. 

 52 — 54, and their characters certainly justify the assump- 

 tion that they are but slight modifications of ordinary stellate 

 cells. It appears to me that the calcoblasts, at any rate in the 

 case of large spicules, must be amoeboid, for, unless they 

 be so, I cannot understand how the spicules can increase 

 uniformly in thickness. I have already suggested (9) that 

 there are probably primary and secondary calcoblasts, the 

 former being mother-cells within which spicules are formed, 

 and the latter cells which apply themselves to the surfaces of 

 already formed spicules and increase the thickness of the latter. 

 The cells represented in figs. 52 — 54 are probably secondary 

 calcoblasts; those represented in figs. 44 — 47 may be primary. 



Minchin (32), in the case ofLeucosolenia, found the tri- 

 radiate spicules to have a nucleus at the extremity of each ray, 

 and a fourth one at the confluence of the rays. As there are 

 four of these nuclei they probably indicate the presence of 

 secondary calcoblasts, for we can hardly suppose that the 

 spicule is originally formed by more than one cell. 



We come now to the subdermal gland-cells, which Bidder 

 (24) regards as the> '^ pendeut cell bodies'' of flask-shaped 

 ectoderm- cells. These cells may occur beneath both the 

 gastral and the dermal surfaces of the sponge. In Grantia 

 labyrinthica they are more plentiful beneath the gastral than 

 beneath the dermal surface, a fact which I associate with the 

 peculiar shape of the gastral cavity, which causes its surface 

 to be almost or quite as much exposed as the dermal surface. 

 A situation beneath the dermal surface, however, appears to be 

 their normal one, and they are rare elsewhere. To my descrip- 

 tion of these structures in Grantia labyrinthica I have 

 little to add. I have found them especially well developed in 

 Grantiopsis cylindrica (fig. 56), where the connection 

 with the surface is very long and slender. In Leucandra 

 phillipensis I have detected them around the inhalant 



' Compare with these the "conjectural calcoblast," figured by Polejaeff 

 (8J in his Leuconia multiformis. 



