SILICIOUS AND HORNY SPONGES WILSON. 293 



of most minute spheres (" winziger Kugeln") which are held to- 

 gether by a cement substance, and round each mass is a follicular 

 epithelium. Keller thinks it likely that the masses (or only the 

 cement substance?) are made up of spongin. 



A conspicuous feature of a radial section through the Albatross 

 sponge is a layer of bodies having about the same distribution and 

 general appearance as the above. My material is doubtless better 

 preserved than Keller's, and I find the masses to be groups or nests 

 of spheruliferous cells ("cellules spheruleuses " of Topsent). The 

 data are as follows: As seen in radial sections the masses are ar- 

 ranged in a single layer outside the fibrous stratum of the cortex, 

 between it and the actual surface. The layer is interrupted by the 

 radial megascleres and cortical canals; otherwise it is almost con- 

 tinuous. The masses, 100-160 \i in diameter, have, except near the 

 surface, a sharp boundary. The lower and major part of each mass 

 is more or less spheroidal in shape and is outlined by a thin but 

 fairly conspicuous layer which is not a special cellular follicle but 

 only a condensation of the surrounding mesenchyme. The masses 

 are yellow in color. 



Each of these problematical masses is a densely or loosely packed 

 group of spheruliferous cells. The cells are 8 y. in diameter, and 

 when stained with haematoxylin show a central nucleus. The cell 

 body is entirely filled with minute spherules about 1 \i in diameter 

 and yellow in color. A few spheruliferous cells of this kind may 

 also be found scattered in the ectosome. Close to the surface of the 

 sponge at the level of the triaene cladomes the nests of spheruliferous 

 cells meet and fuse with one another in irregular fashion. This is 

 best seen in tangential sections. Distal to the cladomes there is a 

 very thin layer of the minute yellow spherules themselves. In this 

 situation the spherules are no longer aggregated in cell groups. 

 They must have broken out of the cells which produced them. They 

 are best seen in thin tangential sections of the surface, where they 

 appear, in places at any rate, as a single and continuous layer, on 

 which are scattered the minute asters. The function of this super- 

 ficial layer of spherules can only be guessed at. 



It may be noted that a closely crowded superficial layer of granu- 

 lar cells has been recorded in other Stellettidae — as, for example, 

 in Stelletta crassiclada (Lendenfeld, 1906, p. 281). 



Genus STELLETTA O. Schmidt (1862). 



KUHrttd (). Schmidt, 1S<;l\ p. 4<>. 



With or without a fibrous cortex. Microscleres are euasters of two 

 kinds, one forming a dermal layer from which the other, the larger, 

 is excluded. 



Sollas (1888, p. 150) restricts the genus to forms with a fibrous 

 cortex and well differentiated chones. Lendenfeld (1903, p. 33) 



