110 GENERAL BIOLOGY 



the alimentary canal in a sort of ring or collar. (See 

 fig. 39.) 



Skeletal Structures. Endoskeleton. In the Pro- 

 tozoa the cell-body is so small that it holds together, 

 as a rule, of its own viscosity. Yet even here, in 

 those groups in which the protoplasm is most foamy 

 and watery, we find a skeleton or supporting frame- 

 work developed which gives rigidity and form to the 

 protoplasm itself. Sometimes, as in the sun-animal- 

 cules (Heliozoa), this skeleton attains great com- 

 plexity. In some groups of Protozoa it is composed 

 of silica, in others, of lime. In the corals, the soft 

 " polyps " secrete so much skeleton that the animal 

 portion becomes but a fraction of the whole. In the 

 sponges the skeleton is made up of innumerable 

 needles or spicules of lime or silica. In one group of 

 sponges the skeleton is composed of a network of 

 tough fibers of peculiar composition. When the 

 protoplasmic portion of the sponge has been soaked 

 out, this skeleton forms the " bath-sponge " of 

 commerce. 



In the larger animals the weight of the body would 

 make it impossible for a constant form to be main- 

 tained were it not for the fact that there exists an 

 internal framework or scaffolding to which the softer 

 parts are attached. Irr the vertebrates this is usually 

 made up of mineral elements, the bones, although in 

 some of the lower fishes the skeleton is composed of 

 cartilage (gristle) instead of bone. This bony frame- 

 work may be divided into two systems, one under- 



