CH. IV] LEUCOSOLENIA 85 



partly closed by a perforated membrane. This opening, which at 

 first sight recalls the mouth of Hydra, is really used for a quite 

 different purpose. It is an efferent opening (Lat. effero, I 

 carry out) and from it the water which has passed through the 

 animal is expelled. Water enters the internal cavity through a 

 multitude of very fine pores in the walls of the tube (Fig. 37, and 

 1, Fig. 39) : it is the universal presence of these pores which gives 

 the name For if era to the group. 



The wall of the tube is made up of two layers, but we must 

 guard ourselves against rashly comparing these with the layers of 

 the body wall of Hydra, and hence it is better to avoid the names 

 ectoderm and endoderm and adopt the terms dermal and gastral 

 layers. 



FIG. 36. View of a branch of Leucosolenia sp., showing the sieve-like mem- 

 brane which stretches across the osculum. The lower part of the sponge 

 shows spicules only x 10. From Minchin. 



1. Osculum with sieve-like membrane stretched across it. 



The dermal layer consists of flat cells which cover the external 

 surface and extend for a short distance inside the osculum, and of 

 cells termed amoebocytes from the resemblance of their movements 

 to those of an Amoeba: the whole of the rest of the tube and the 

 stolons are lined by a tissue consisting of peculiar cells called 

 choanocytes (Gr. x '^ ^ a funnel; KV'TOS, a hollow vessel), or 

 collar cells, which alone constitute the gastral layer (3, Fig. 37 

 and Fig. 38). Each of these is cylindrical and provided with a 

 funnel-shaped transparent rim called the collar, .turned towards 

 the cavity of the tube. The collars of adjacent cells are not 



