A SIEVE-LIKE MEMBRANE IN LEUCOSOLENIA. 257 
Hydroid colony. This is, I believe, the commonest method of 
the formation of new oscula. In fact, I believe that in the 
other species of Leucosolenia mentioned above, it is the 
only way in which new oscula are formed, and explains the 
difference in the mode of growth between the two sponges. 
Blind diverticula of the tube composing the sponge occur very 
commonly, and I have observed many such. This method of 
formation of the osculum is essentially similar to the forma- 
tion of the primitive osculum in the young sponge after the 
metamorphosis from the larval condition, when the osculum 
always arises as a breaking through of the gastral cavity to the 
exterior. Now in this mode of oscular formation a sieve 
membrane similar to that here described might be formed in 
one of two ways. The simplest method would be by the gastral 
cavity breaking through to the exterior in not one, but several 
places. The result would be the formation of a sieve-like 
membrane of two layers, in which the inner layer was endo- 
derm, the outer ectoderm. Or secondly, after a simple wide 
opening was formed, a ring-like ingrowth of the margin of the 
osculum might take place towards the centre of the aperture, 
forming akind of diaphragm, which, after becoming second- 
arily perforated, would form a sieve membrane in which both 
layers of cells might be ectoderm. I strongly believe myself, 
though I have no direct observations to support my views, that 
the sieve membrane here described arises in the first method 
suggested above, as a breaking through in several places of the 
gastral cavity to theexterior. In fig. 3 is represented one of a 
series of sections through an osculum, which, besides being of 
very small size (116 m in diameter, vide supra), is further 
marked out by the fact that its membrane has no open- 
ing, either in this or in any of the sections, of which my 
series is perfectly complete. I have also another series of 
sections through a precisely similar osculum, in which there is 
no trace of an opening in the membrane. In the osculum 
represented in fig. 3, of which my sections are very satisfactorily 
preserved and stained, I noticed three other points. First, I 
could see no pores at all in the wall of the oscular tube. 
