Descriptive Zoology. 



pores do not lead directly into the main cavity (as shown in 

 Figs. 176 and 177), but do so indirectly. The cells bearing 

 the flagella, instead of lining the main cavity, are here in 

 the radiating canals that open into the main cavity, and 



receive the water through cross 

 openings from the incurrent 

 canals. As in the simpler 

 sponge, there are supporting 

 spicules in the walls. 



The Highest Sponges. The 

 general plan of structure of the 

 highest sponges may be illus- 

 trated by Fig. 178. The most 

 noticeable peculiarities are two : 

 ( i ) The cilia are limited to cer- 

 tain cavities, or enlargements, 

 along the course of the passages 

 from the outside to the main 

 cavity. These cavities are called 

 the " ciliated chambers." (2) The 

 whole sponge is no longer a 

 vase or cylinder, but a mass, in 

 which the cavities are less con- 

 spicuous. This is due to the 

 increase of the middle layer, or 

 mesoderm, a gelatinous mass of 

 cells. As before, the outer layer 

 is the ectoderm, and the lining 

 the endoderm. 



Kinds of Skeletons. Sponges 

 may be classed, according to the nature of their skeletons, 

 into three groups : 



i. Calcareous sponges, whose skeletons consist of 



FIG. 176. 



ONE OF THE SIMPLEST SPONGES, 

 Calcolynthus primigenius. 



(After Haeckel.) 



A part of the outer wall is cut away to 

 show the inside. From Jordan and 

 Kellogg's Animal Life. 



