BRITISH SPONGES 



117 



respiration to go on with equal freedom in the interior. The mode 

 of feeding of the sponge is very similar to that of the protozoa. 

 Organic particles that are carried into the body-cavity, on coming 

 in contact with the cells of the internal layer, are absorbed into 

 their protoplasm by which they are digested. Thus the sponge 

 may be compared to a mass of protozoon cells, all united into 

 a common colony by a more or less perfect coalescing of the 

 cell-substance, some of the units being modified in structure 

 for the performance of definite functions. The air and food 

 absorbed by any one cell may pass readily into the surrounding 



FlG. 66. DIAGRAMMATIC SECTION OF A POETION OF 

 A COMPLEX SPONGE 



cells, and thus each one may be said to work for the common 

 weal. 



The description just given applies only to the simplest of the 

 sponges, and we have now to learn that in the higher members 

 of the group the structure is much more complicated. In these the 

 surf ace -pores are the extremities of very narrow tubes which 

 perforate both layers of the body-wall and then communicate with 

 wider tubes or spaces within, some of which are lined with the 

 ciliated cells above described. These spaces, which are sometimes 

 nearly globular in form, and often arranged in groups with a 



