SKELETONS OF INVEETEBEA.TES 



253 



jointed skeleton whose central structure is the flexible 

 column of bones called vertebrae. 



Skeletons of Invertebrates. — In certain of the protozoans 

 the cytoplasm secretes a layer of substance which, collect- 

 ing over the surface of the animal, forms a protective 

 outer membrane or wall. In certain- cases this membrane 

 becomes the seat of deposit for lime or siliceous material, 

 and this deposit gives it a firm character which increases 

 its power of support and protection. (See Fig. 101.) 



In the sponges certain cells in the body produce spicules 

 of lime, or silica, or spongin, which are embedded in the 



Fig. 102 — Skeleton of horny sponge. 



walls of the animal and form an interlacing network. The 

 ordinary sponge of commerce consists of the spongin 

 skeleton of such anim.als with the animal parts removed. 



