260 PRACTICAL LESSONS IN SCIENCE. 



minute animals are the chief constituent of thick beds of lime- 

 stone rocks, as the chalk cliffs of England and nummulitic lime- 

 stone of France. They are exclusively marine animals, and good 

 specimens of their shells may be obtained by shaking a sponge 

 that has not been treated with acid. 



The radiolaria are marine animals whose body contains a 

 capsule in which are oil globules, albuminous bodies, and some- 

 times concretions, with vacuoles and granules. They secrete a 

 silicious skeleton or shell. Their skeletons have given rise to ex- 

 tended rock formations. 



If some vegetable or animal substance be soaked in water for 

 a few days, the water will be found to contain a large number of 

 different kinds of microscopic animals called infusoria. Among 

 them may be vorticella, paramoecium, stentor and others. One 

 group of the infusoria is especially interesting on account of their 

 close relationship to some of the alga3. They consist of a colony 

 of cells united by a gelatinous substance. In the inactive stage 

 they possess a cellulose membrane, exhale oxygen and have an 

 abundance of chlorophyll, and reproduce by simple division. 

 During the active stage the cells give rise to daughter colonies. 

 They are called volvocinidse, and the most common is volvox 

 globater. Some of the infusoria swim freely by means of their 

 cilia; others, fixed in one locality, create currents in the water by 

 the action of their cilia, thus bringing more food within their 

 reach. The usual mode of reproduction is by self-division, some- 

 times by budding and sometimes by conjugation, the nucleus 

 and nucleolus taking an important part in the process. 



The Coslenterata. Among the coelenterata we find organs and 

 tissues composed of cells ; they have external and internal epithe- 

 lium, calcareous and silicious structures, with muscles, nerves and 

 sense organs. To this division belong the sponges, which are 

 composed of a contractile substance made up of amoeba-like cells 

 supported by a framework of horny fibers, or of calcareous or 

 silicious spicules. The sponge is traversed by numerous canals, 

 which open on the surface by larger or smaller openings. At 

 numerous places in the sponge there are little cavities or pas- 



