A SIEVE-LIKE MEMBRANE IN LEUCOSOLENIA. 205 
opening just above where the collar cells end (figs. 1, 2, 8, 5, 
13). Having found no mention of such a membrane in the 
sponge literature! I proceeded to investigate it further, intend- 
? Haeckel, in his classical monograph ‘ Die Kalkschwimme,’ Bd. i, p. 267, 
describes, under the name of “‘ Mundhaut oder Oscular-membran,” a structure 
in Sycons and Leucons which resembles somewhat from his description (there 
are unfortunately no figures) the membrane here described. In the Ascons 
**ist mir ihre Existenz iiberhaupt noch zweifelhaft.’’ He writes, ‘“‘ Die Oscular- 
Membran ist eine diinne, keine Spicula enthaltende Lamelle des Syncytium, 
welche inwendig von der Basis (dem aboralen oder unteren Rande) des 
Riissels oder des Peristom-Kranzes ausgeht. Bei weit geoffnetem Mundcanal 
wird sie (durch Retraction in das Exoderm) entweder ganz unsichtbar, oder 
bleibt bloss als ein ganz schmaler Ring stehen. Bei vollig geschlossenem Mund- 
canal hingegen bildet sie eine sehr zarte transversale Scheidewand, welche 
senkrecht auf der Langsachse des Magen steht.” This membrane of Haeckel’s 
has therefore quite a different structure from that which I describe here, 
but it has precisely the same relations to the osculum, and may well be homo- 
logous with it. It is to be hoped someone will give us before long a fuller 
description (with figures) of this “ Oscular-membran.” My friend, Mr. G. P. 
Bidder, has directed my attention to a passage in the works of Dr. Grant 
(‘ Edinburgh Philosophical Journal,’ xiii, 1825, p. 381), quoted in Johnston’s 
‘ British Sponges and Corallines,’ p. 51. ‘ When we cut,” says Dr. Grant, 
“a thin piece off the surface of a living sponge and look down through one of 
its pores with the reflecting microscope, we perceive, immediately beneath 
the projecting spicule which defend the pore, a very delicate network of 
gelatinous threads thrown over the entrance of the tube. This piece of 
structure is so fine as to be perfectly invisible to the naked eye; it consists 
of five or six threads, which pass in from the sides of the tubes to be con- 
nected with a central mesh, so that there are six or seven meshes thus 
formed; and while this soft apparatus is beautifully defended by the pro- 
jecting spicula of the pore, it serves still further to guard the interior of the 
animal from the smallest particles of sand, or the minutest visible animalcules.” 
Since Grant distinguished clearly, both in this work and in others, between 
* pores’ and “ fecal orifices,” this network of his can have nothing to do 
with the membrane I describe here. But Johnston seems to have taken this 
description as applying to the oscula (T. c., p. 58, foot-note), and reminds 
his readers that Grant’s description can apply to the oscula of one or two 
species only. He adds that “in general the oscula are merely simple or com- 
pound outlets, without any protective net over the orifice or in the funnel; 
and indeed it can rarely be seen except in newly formed oscula before the 
fibres of the sponge have been broken away by the effluent current ”’—a most 
noteworthy statement. 
