ANIMAL FORMS 



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



water. 



27. 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 

 a support the larger 

 globular or branched 

 forms could not ex- 

 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 fiat 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- 



FIG. 14. Portion of wall of sponge, showing three 

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

 of collared cells ; m, middle layer, consisting of 

 irregular cells, among which are the radiate spic- 

 ules and egg-cells. 



FIG. 15. Different types of sponge spicules. 



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. 15). These occur 

 everywhere throughout the body, firmly bound together 



