THE STRUCTURE AND LIFE-HISTORY OF SPONGES. 225 



The canal system of different sponges is veiy differently 

 developed. The form described above is that of the most highly 

 developed sponges, the Chondrospongice and Gornacuspongice. 

 The entodermal as well as the ectodermal epithelia of sponges 

 always consist of one single layer of cells only. In this character 

 the main difference between sponges and higher Coelenterates is 

 to be found. In the latter the epithelia always consist of 

 several laj^ers of cells, from the lower subepithelial layer of which 

 all the organs are developed. 



The epithelial cells of sponges are of two kinds onl}' — flat 

 pavement-cells and collared cells. Each epithelial cell possesses a 

 cilium. In the most simple sponges, the sac-shaped Asconida, the 

 ectoderm consists of a single layer of flat cells on the outer 

 surface of the sponge, and the entoderm forms a single layer of 

 collared cells on the inner surface of the sponge ; flat entodermal 

 cells do not occur in these sponges. Whilst in all, even the most 

 highly developed forms, the ectodermal epithelium invariably 

 consists of flat pavement-cells, we find in all sponges, with the 

 exception of the above-mentioned group, two kinds of entodermal 

 cells — collared cells and flat entodermal pavement-cells. The 

 collared cells are confined to the ciliated chambers, whilst the 

 whole exhalent canal system and the oscular tube are clothed 

 with a low epithelium of flat pavement-cells. 



In the Hexactinellidce the collared cells are connected with 

 each other by basal processes ; but, as a rule, they are isolated. 

 Their shape is very constant — long, cylindrical, with a long, 

 projecting, more or less cup-shaped, hyaline collar, which 

 appears as a marginal extension of the free end of the cell. 

 The long flagellum is inserted in the middle of the cup formed 

 by this collar. Whilst the collared cells always appear to be filled 

 with readily stainable granular protoplasm, the hyaline plasma 

 of the low, flat cells is confined to a mass surrounding the 

 nucleus, from which protoplasm-threads radiate to the proto- 

 plasmic coating of the cell-wall. It seems that the only 

 difference between the ectodermal and entodermal pavement- cells 

 is their difference of height. The former are mostly slightly 

 lower than the latter. 



The function of the pavement-cells does not appear at first 

 sight to be a very important one, because these cells are not com- 

 pletely filled with protoplasm. The collared cells in the ciliated 



