CLASSES OF ANIMALS 449 



There are more species of animals than there are of plants, 

 and there are also more main divisions, or branches. In the 

 following descriptions only the more important groups are 

 described/ 



327. Protozoa. The protozoa live for the most part in natu- 

 ral waters, but many species are parasitic upon plants, others 

 upon animals. 



Some species have delicate lime cases ; millions of these, fall- 

 ing to the bottom of the ocean and deposited in past ages, make 

 up the chalk cliffs and lime deposits of today. Another group, 

 bearing sandy shells, have in a similar manner contributed to 

 vast deposits that are used today as grinding or rubbing mate- 

 rials. Compare Barbados earth to the fuller's earth deposited 

 from plant skeletons. 



We have already seen the importance of the malarial Plasmo- 

 dium and of the parasite causing yellow fever (see pages 322- 

 325). Sleeping sickness and probably smallpox are also caused 

 by protozoan parasites in man : many of the diseases of cattle 

 and other domestic animals — Texas fever, for example— are due 

 to protozoan parasites. A number of relatives of the ameba are 

 found in the intestines of higher animals, and these sometimes 

 cause serious disorders. The pebrine of silkworms, the problem 

 that aroused Pasteur's interest in the causes of disease, is also 

 due to a parasitic protozoon. 



328. Porifera. The '"pore-bearing" animals consist essentially of a 

 skeletal framework built up by the secretions of the living cells and sup- 

 porting layers of cells that are substantially alike (see Fig. 43). There 

 is very little division of labor, but there is a great deal of common activ- 

 ity. The sponges of commerce are made of the horny skeletons of the 

 sponge colonies (see page 430). 



328. Coelenterates. From a practical point of view the corals are 

 perhaps the most important. It is through the growth of these colonial 

 animals and the laying down of their lime skeletons that coral reefs and 

 islands have been built up. Some of the coral structures are very beau- 

 tiful ; the pink coral of the Mediterranean is most highly prized and 

 most extensively used for ornament. 



^For the plan of classification see Chapter VII. 



