ANIMAL STUDIES 



is absorbed by all parts of the body in contact with the 



water. 



37. Skeleton of sponges. When it is remembered that 



the protoplasm composing the cells of the sponge has about 



the same consistence 

 as the white of egg, 

 it will be readily un- 

 derstood why the 

 greater number of 

 sponges possess a skel- 

 eton. Without such 



FIG. 17.-Portion of wall of sponge, showing three a suppor t the larger 

 layers, e, outer layer ; i, inner layer, consisting rr 



of collared cells ; ra, middle layer, consisting of globular Or branched 



irregular cells, among which are the radiate spic- f ormg could not CX- 

 ules and egg-cells. 



ist, and even in the 



smaller members there would be danger of a collapse of the 

 body walls and consequent stoppage of the food supply, 

 owing to the closure of the pores. So in all but a very few 

 thin or flat forms a skeleton appears in the young sponge 

 almost before growth 

 has fairly begun, and 

 this increases with the 

 body in size and com- 

 plexity. It is formed 

 by the activity of the 

 cells of the middle layer, 

 and may be composed 

 either of a lime com- 

 pound resembling mar- 

 ble, or of flint, or of a 

 horn-like substance resembling silk, or these may exist in 

 combination in certain species. When consisting of either 

 of the first-named substances it is never formed in one 

 continuous piece, but of a vast multitude of variously shaped 

 crystal-like bodies termed spicules (Fig. 18). These occur 

 everywhere throughout the body, firmly bound together 



m f\ -^ 



FIG. 18. Different types of sponge spicules. 



