STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 345 
It is obvious that Sollas’s membrane divides the cavity of 
the flagellated chamber into two portions, (1) a central portion 
into which the flagella project, and (2) a peripheral portion 
lying between the collared cells. It is farther clear that the 
stream of water kept up by the movements of the flagella can 
pass only through the central portion of the chamber. It may 
give access to the peripheral space by means of gaps between 
the collared cells around the inhalant aperture, but as it has 
no means of egress this would be of no consequence. In 
short, the membrane may serve to facilitate the flow of the 
water by diminishing the friction, for it is evident that the 
water will flow more easily over a smooth membrane than if it 
had to run in and out between the collared cells. It has been 
pointed out to me by Sir James Hector that Sollas’s membrane 
may also play some part in determining the direction of the 
current of water, but this is a mechanical problem which I am 
at present unable to solve. 
Now that attention has once been directed to its existence 
this membrane will probably be found to be very generally 
present in Sponges. I may state here that I have already 
detected it in H alichondria panicea, of the minute anatomy 
of which species I hope to be able to give an account before 
very long, so that it is now known to exist in three groups, 
Tetractinellida, Ceratosa, and Halichondrin a. 
(4) The General Mass of Mesoderm in which the 
Chambers and Canals are Embedded. 
The flagellated chambers are pretty closely packed together 
in the choanosome, and together with the various branches of 
the canal system make up the greater part of its bulk. Hence 
the amount of fundamental or ground tissue in which they 
are embedded is not very great. What there is is packed full 
of minute, highly refringent granules (figs. 6, 7, 8, 9, 10), 
fairly evenly distributed through it, and resembling in size and 
general appearance the intracellular granules of some of the 
forms of tissue already described (e. g. the fibrous cells which 
form the inner layer of the embryo capsules). 
