OSCULA AND ANATOMY OF LEUCOSOLENTA CLATHRUS. 485 
derm. But the muscle-cells described by Topsent and in this 
paper make it, I think, to say the least, extremely probable 
that all muscular cells in sponges are of epithelial origin. 
Dr. von Lendenfeld has published,! at divers times and in 
divers places, a classification of the Coelenterata into Mesoder- 
malia (sponges), in which the principal organs are derived 
from the mesoderm; and Epithelaria (other Coelenterates), in 
which the principal organs are derived from the epithelia. 
What are the principal organs of a sponge? I presume 
the ciliated chambers, skeleton, genital products, and the 
various kinds of muscle-cells, gland-cells, nerve-cells, &. The 
skeleton certainly appears to be mesodermal, as far as we can 
judge, and perhaps also the genital cells. On the other hand, 
the ciliated chambers are almost certainly endodermal, and 
the muscle-cells of epithelial origin. There does not appear 
to be the slightest reason why the nerve-cells, so often de- 
scribed by von Lendenfeld, should (if they exist) be of meso- 
dermal and not of ectodermal origin, as in other groups of 
animals ; and the same may be said of their gland-cells. Thus it 
appears that the only principal organs of a sponge which can 
with any certainty be said to be of mesodermal origin are the 
connective-tissue system and the generative elements. 
To return, however, to our oscula. We have in this 
sphincter a mechanism for closing the osculum, and in the 
sieve membrane over the oscula of Leucosolenia coriacea 
we have, I do not doubt, a strncture which can be employed 
for a similar purpose, since Auloplegma forms of the latter 
sponge are so common. [I look upon this as a good instance 
of two structures physiologically similar, but morphologically 
quite different. In my paper on the sieve membrane? I ex- 
plained it as probably arising as a breaking through of the 
gastral cavity to the exterior in several places during the 
formation of the osculum, and hence as consisting of ecto- 
1 © Monograph of the Horny Sponges ’ (London, 1889), p. 889; ‘ Proc. Zool. 
Soc., London, 1866, p. 566; ‘ Biol. Centralbl.,’ ix, 1889, pp. 1]3—127, &. 
* “Note on a Sieve-like Membrane across the Oscula of a Species of 
Leucosolenia, &c.,” ‘ Quart. Journ. Micr. Sci.’ (n.s.), Part 2, January, 1892. 
