SPONGES 209 



2. Sponges are said to possess a skeleton, but the term The sponge 

 is employed in a very loose sense. We mean that there 

 are fibers or spicules which give the structure its stiffness 

 and prevent it from falling to pieces even when all the 

 living material has been removed. In an ordinary bath 

 sponge we see this skeleton, consisting of a horny sub- 

 stance called spongin, which in life is merely the frame- 

 work of the animal. One group of sponges has a skele- 

 ton made of calcareous or limy spicules, while others 

 have the spicules siliceous or flinty. This property of 

 secreting different materials, limy or flinty according to 

 the species, is found also in Protozoa. The hornlike 

 substance, spongin, is said to be allied to silk. The 

 spicules resemble little crystals in form, and it is charac- 

 teristic of the relatively unorganized and vegetable-like 

 growth of the sponge that these units are scattered 

 through the substance, instead of being articulated to 

 form a definite mechanical unit comparable to the skele- 

 ton of a vertebrate. It must be said, however, that the 

 structure of the whole animal is often complicated and 

 beautiful, especially in the flinty forms. 



There is no special nervous system, and therefore the 

 actions of the cells are largely independent of one an- 

 other, as though they were distinct individuals. It is 

 well to remember in this connection that even in our- 

 selves, with our brain and highly organized nervous 

 system, the white blood cells behave essentially as in- 

 dependent units. 



