THE NEW EVOLUTION 



the body mass encrusting, cushion-shaped, cup-shaped, saucer- 

 shaped, globular, tubular, rod-shaped, leaf-like, trumpet-shaped, 

 fan-shaped, mushroom-shaped, lobed, branched or digitate, 

 sessile or raised on a long stalk, etc. Almost all sponges contain 

 a supporting skeleton which may be limy, or silicious, or horny, 

 or silicious and horny, and is usually exceedingly complex. 

 Nearly all sponges are marine, living at all depths. Some types 

 are especially characteristic of deep water. But a few kinds of 

 sponges live in fresh water. All sponges when alive possess a 

 strong and disagreeable odor. The common bath sponge is the 

 cleaned skeleton of one of the horny sponges, and the "Venus' 

 flower basket" is the cleaned skeleton of one of the silicious 

 sponges. (Figs. 1x1-114, p. 185, development.) 



Protozoa — the single-celled animals. — Very small, usually 

 microscopic, animals, forming an enormous group, which is the 

 most highly diversified of all the groups in the animal kingdom. 

 More or less familiar examples are the foraminifera, to which we 

 owe the great chalk deposits, the radiolarians, the infusorians, 

 the gregarines, the sporozoans, etc. They are found everywhere 

 in the sea and in fresh water, more or less throughout the bodies 

 of all other creatures, and everywhere on land. In a special 

 drought-resisting resting stage, many kinds are blown about over 

 the land and lodge in quantities even on the topmost twigs of 

 the tallest trees. If bits of bark or hay or dead leaves be placed 

 in water they will appear in a day or two, and soon the water 

 is swarming with them. Some of those that can be secured from 

 the bark of fire logs are large enough to be seen with the naked 

 eye, and many may be seen with a low power lens. Very many 

 live upon or within the bodies of other creatures. Crustaceans 

 are sometimes almost completely covered with a veritable forest 

 of stalked forms, while others wander about over the surface of 

 sea-urchins and other sea animals. Our bodies always contain 

 many thousands of various harmless kinds. But very many are 

 dangerous parasites, causing malaria and other diseases in man, 

 Texas fever in cattle, etc. On the other hand, some are indis- 

 pensable. Thus in the case of the termites or white-ants the 

 cellulose swallowed is broken up not by the digestive processes 

 of the insects, but by the intervention of various protozoans in 

 their intestinal canal, and the termites digest the substances 

 thus made available for them. Reproduction in the protozoans 



[1.66] 



