REPORT ON THE TETRACTINELLIDA. 
315 
(PL XXXIII. figs. 11, 12), is continued into the fenestrated membrane, which crosses 
the aphodal face of the chamber, as in nearly all Tetractinellida ; it is here very con- 
spicuous, so that I first discovered its existence in this sponge. 
The fenestrse are round or oval, and from 0'004 to O'OOG mm. in diameter ; sometimes 
a small, round, deeply stained body is seen occupying their centre. This appears to me 
to be the body of one of the choanocytes detached from the w^all of the chamber, and 
drawn by the contraction of the collum into the middle of the fenestra. 
Coating the desmas of the skeleton is a thin lamella, which stains with hsematoxylin ; 
it is to be frequently met with detached from both the desma and the surrounding 
sarcenchyma. It appears to be fragile, breaking readily into polygonal fragments. It 
was eagerly searched for associated cellular elements or scleroblasts, but as a rule no 
structure of any kind could be discerned in it. In a few" instances, however, the appear- 
ances represented in the illustrations (PL XXXIII. figs. 15, 15a-d) were observed; they 
were so rare and exceptional, how^ever, that I hesitate to attach any importance to them. 
That the lamella is in some manner connected with the formation of the desma appears 
however to admit of little doubt, and it is just possible that it may represent an exuda- 
tion from surrounding but hypothetical scleroblasts that has since become hardened by 
treatment. It is also possible that the lamina represents a stratum of organic matter 
destined to receive a deposit of silica, and so to add to the thickness of the desma, and 
in this case the apparent cellular structures associated with it may be exhausted sclero- 
blasts, though, judging from the homogeneous nature of the outer layers of the desma, 
this seems unlikely. 
Demus II. Rhabdosa. 
Hoplophora in which the ectosomal spicules are microstrongyles or modified micro- 
strongyles (discs). The desmas are monocrepidial. 
Family II. Scleritodermida:. 
Rhabdosa in which the ectosomal spicules are microstrongyles, and the other micro- 
scleres sigmaspires. 
Genus 1. Scleritoderma, 0. Schmidt. 
Scleritodermidse of plate-like form, bearing simple oscules on one face, and simple 
pores on the other. 
