26 AETHUR DENDy. 



figures a " conjectural calcoblast " attached to the outside of a 

 spicule of Leuconia multiformis. This cell has the form 

 of an ordinary stellate mesodermal cell, and such, so far as 

 structure is concerned, I believe the calcoblast to be. In the 

 horny sponges the corresponding spongoblasts, although some- 

 what specialized in form, are clearly only slight modifications 

 of stellate cells, as I have elsewhere shown (2). In the cal- 

 careous sponges the calcoblasts have acquired the function of 

 secreting carbonate of lime without undergoing any correspond- 

 ing modification in form. It should, however, be borne in 

 mind that there are probably two kinds of calcoblasts, primary 

 and secondary. The primary calcoblasts are the mother-cells 

 in which the spicules originate, and the secondary calcoblasts 

 are the cells which secrete additional layers of calcareous matter 

 around the spicule after it has been formed. In the same way I 

 have no doubt that there are primary and secondary silicoblasts.^ 

 The slime- or cuticle-secreting cells have, so far as I am 

 aware, not hitherto been observed in calcareous sponges, 

 although well known in the Keratosa through the researches of 

 von Lendenfeld. In Grantia labyrinthica the gland-cells 

 in question occur in a single layer just beneath the epithelium 

 of the surface of the sponge. They are very distinct and 

 plentiful on the gastral surface (fig. 26, gl. c), but less so on 

 the dermal. Each gland-cell consists of an irregular, granular, 

 nucleated body, which closely resembles an ordinary amoeboid 

 cell, but which may be seen under favorable conditions to be 

 connected with the overlying epithelium by one or more pro- 

 cesses. Fig. 26 shows the layer of gland-cells beneath the 

 gastral epithelium, and fig. 34 is a more highly magnified 

 drawing of an individual gland-cell and its surroundings, as it 

 appeared under a Zeiss F objective and ocular 4. At the par- 

 ticular spot figured an irregularity in the surface caused the 

 epithelium to be cut somewhat tangentially, while the gland- 

 cell itself was cut vertically ; the connection between the two 

 is, however, very well shown. Four processes are shown in this 



' For an illustration of a primary siliooblast the student is referred to the 

 'Report on the "Challenger" Monaxonida,' pi. xxi, flg. 13. 



