30 ELEMENTS OF BIOLOGY. 



simple or compound, and have the power of throwing out and retracting 

 temporary prolongations of the body-substance (" pseudopodia "). A 

 mouth generally, if not universally, absent. Ex. Sponges. 



CLASS C. INFUSORIA (Infusorian Animalcules). Protozoa mostly 

 with a mouth and short gullet ; destitute of the power of emitting 

 pseudopodia ; furnished with vibrating hair-like processes (cilia) or con- 

 tractile filaments ; the body composed of three distinct layers. Ex. 

 Bell-animalcule. 



SUB-KINGDOM II. CCELENTERATA. 



Animals whose alimentary canal communicates freely with the general 

 space included within the walls of the body, so that the "body-cavity" 

 comes to communicate with the outer medium through the mouth. 

 Body composed of two fundamental layers or membranes, an outer 

 layer or " ectoderm," and an inner layer or " endoderm." No central 

 organ of the circulation or distinct blood-system ; in most no nervous 

 system. Skin furnished with microscopic stinging organs or "thread- 

 cells." Reproductive organs in all, but multiplication often by non- 

 sexual methods (figs. 5 and 8). 



A B 



Fig. 8. Ccelenterata. A Hvdra vulgaris, the common fresh-water Polype (after 

 Hincks). B Diagrammatic section of a Hydra. 



CLASS A. HYDROZOA. Walls of the digestive sac not separated from 



