558 STUDIES ON SPONGES, 



Inhalent pores covered with the usual sieves are found throughout 

 the surface of the Pseudogaster. These are perfectly similar to 

 the inhalent pores on the outer exposed surface of the sponge. 



These extensive lacunae are not in direct communication with the 

 outer world, we find moreover, at the distal terminations of the 

 tubes in the digitate processes very fine and perforated membranes 

 (fig. 1), dividing them from the water without. 



It is easy to observe these membranes, which occasionally attain 

 a size of 200 square millemeters, and we find that the pores in 

 them may be so wide open that bridges as wide only as the pores 

 themselvs are left between them. In other cases again the 

 pores ai'e found to be entirely closed, and every intermediate 

 stage in the dilatation of the pores can be observed. 



Two adjacent pores are always dilated nearly to the same width. 

 It is never observed that one pore is nearly closed and the next one 

 wide open. Generally we find the pores in these membranes on 

 the terminations of some of the processes wide open, those in 

 others nearly or completely closed. Occasionally I have also 

 observed that the pores at one end of one and the same membrane 

 are dilated much more than at the other. This however, is rare. 

 If the sponge is killed rapidly by immersion in very strong spirits 

 then the pores remain open. If however the sponge dies slowly by 

 exposure to the air, or if it is placed in weak s])irits then the pores 

 are generally found closed. 



Thei-e can be no doubt that the width of these pores is subject 

 to very great variations, and that by means of these the current 

 of water in the lacunae can be regulated by the sponge. 



The physiological value of these vestibule cavities divided 

 from the outer water by membranes, with small pores, which can 

 be dilated and contracted is not quite clear. At the breeding 

 time of the sponge, from September to December (or longer), 

 these cavities contain the embryos which swarm about in them in 

 great numbers. Then they appear as breeding cavities. The 

 mother sponge can let them out or keep them in, according to 

 weather, the pores in the membranes are dilated or contracted. 

 When the water outside is bad, it can be kept from the embryos^ 



