46 AETHUB DENDY. 



water is filtered of the particles, suspended in it by a membrane, 

 formed by the coalescence of the collars, which stretches com- 

 pletely across the current. This coalescence has been figured 

 by Sollas in certain siliceous sponges. The whole evolution 

 of the canal system in sponges consists in increasing the energy 

 of the oscular flow and diminishing the velocity in the flagellate 

 chambers. In these are alike specialised the functions of 

 absorption and propulsion, since to each a low velocity is 

 advantageous. The author believes that the collared cells 

 primitively both ingest and digest the food, the collars having 

 as their function its retention ; digestion is only secondarily 

 passed to the mesoderm." 



It appears almost certain that in some sponges SoUas's 

 membrane completely stretches across the prosopyle, so that 

 the water has actually to pass through it on its way into the 

 chamber. We may compare the passage of the water through 

 Sollas's membrane to the passage of liquids through organic 

 membranes by osmosis, only in sponges it is the flagella of the 

 collared cells which supply the necessary motive power. 



Nevertheless, although Sollas's membrane probably acts in 

 all cases as a trap to catch food particles — a conclusion arrived 

 at independently by Bidder and myself — yet it is not likely 

 that it always stretches completely across the prosopyles or 

 inhalant apertures of the chambers. Whether it does so or 

 not probably depends upon the diameter of the prosopyles. 

 Thus in the Sycons, such as Grantia labyrinthica,^ with 

 large prosopyles, we may safely assume that the membrane 

 ceases around the margin of the prosopyle. In many cases it 

 must, from the nature of the case, be impossible to tell with 

 absolute certainty whether the membrane is continuous across 

 the prosopyles or otherwise ; for even if, as figured by myself 

 in Stelospongos, a gap appears in the membrane, we can 

 never be certain, in dealing with such a delicate structure, 

 that the membrane has not been accidentally ruptured by the 

 mode of preparation adopted. Again, it is quite possible and 

 even probable that temporary gaps frequently make their 



» Yide Dendy, " Studies on the Comparative Anatomy of Sponges," I II. 



