LESSONS IN ZOOLOGY. 261 



sages which are occupied by ciliated cells, which, by the motion of 

 their cilia, keep up a circulation of water through the sponge. 

 Reproduction in the sponges takes place by division, by the 

 formation of gemmules, and by the formation of ova and germ 

 cells. The fertilized ova develop into embryos which are pro- 

 vided with cilia, by means of which they swim about until they 

 find some suitable resting place. Spongilla fluviatilis is common 

 in freshwater ponds; all other sponges are found in saltwater. 

 The sponges of commerce are the horny framework of the 

 sponge, and come mainly from the Mediterranean sea. These 

 sponge colonies vary greatly in form ; one can often find a half 

 dozen distinct forms in the display basket of one drug store. 



The coral polyps, or actinozoa, belong to this division. They 

 consist of a cylindrical or club-shaped tube fixed at one end, 

 having a mouth surrounded by tentacles at the other. They 

 have internal generative organs and usually a calcareous skele- 

 ton. The sexes are usually separate, although hermaphrodite 

 individuals are sometimes seen. The embryos are frequently 

 born alive as ciliated larvae. They also multiply by budding. 

 The body has an ectoderm and endoderm and a mesoderna, which 

 in some cases becomes the seat of calcareous deposits. Corals 

 are radiate animals on the plan of four, five or six. They exist 

 in great variety in the shallow waters of the tropical ocean, 

 building extensive reefs ; there are corals having the form and 

 appearance of a human brain ; there are branching corals and 

 fan corals, etc. They are not only abundant in the seas of the 

 present time but their calcareous skeletons have been the most 

 abundant rock-forming material since the earliest geologic time. 



The Medusse have a soft gelatinous semitransparent bell-shaped 

 body, the margin of which is fringed with stinging tentacles. Near 

 the margin are nerve rings and sense organs as auditory hairs, 

 and in some there is an appearance of eye-spots. The medusae 

 are the jelly-fish or sea-nettles so abundant in many parts of the 

 ocean. Many if not all the medusae are simply the free swimming 

 form of some of the fixed polyps. The polyp gives rise to a bud 

 which becomes a free swimming medusae, while the egg or genera- 



