PROTOZOA IN GENERAL 83 



Another f>;roup is Radiohiria. These have a central perforated cap- 

 sule of chitin and a larji;cr inclosing shell of silica. They also are marine, 

 existing in great numbers in the ocean, and when their shells sink to the 

 bottom they form what is known as radiolarian ooze. When solidified 

 this produces a rock of the nature of flint. These rocks occur in strata 

 several hundreds of feet in thickness. 



Another group of Sarcodina found in fresh water have numerous 

 slender, radiating pseudopodia containing axial threads of chitin. 

 Because of the resemblance of the animal with those radiating pseudo- 

 podia to the sun surrounded by its rays of light, they are frequently 

 termed sun animalcules, and the order to which they belong is called 

 Heliozoa. 



109. Sporozoa. — The Sporozoa (Fig. 29) in their early stages fre- 

 quently are ameboid but in their final stages they lack locomotor organs 

 and form spores. They are parasitic in other animals and are generally 

 transmitted to the host in the spore form. In some cases the life of the 

 individual ends upon the formation of spores but in other cases spores 

 are produced at intervals during the animal's lifetime. 



Among these forms are the gregarines, which exist within the cells of 

 earthworms, cockroaches, and other insects as well as of other inverte- 

 brates and which in their later stages become free in the body cavities of 

 these animals. Those of an order known as Coccidia are found in the 

 liver and intestine of man and other vertebrates as well as in some inverte- 

 brates. Others are found in the blood or muscles of vertebrates or within 

 the cells of fish. One form produces the silkworm disease known as 

 pebrine. Pasteur discovered that this parasite is transmitted from the 

 silkworm moth to the eggs before they are laid and that the caterpillars 

 hatched from these eggs thus become infected. By showing how infec- 

 tion can be avoided he saved the silkworm industry of France at a time 

 when its existence was seriously threatened. 



110. Infusoria. — Infusoria (Fig. 30) occur in both fresh and salt waters, 

 while others are found parasitic in the bodies of higher animals. Para- 

 meciu7n is an infusorian. Opalina is a form which lives in the intestine of 

 the frog. In addition to cilia, infusoria frequently possess midulating mem- 

 branes or cirri, formed by the fusion of numerous cilia. The body may 

 be covered -all over with cilia of approximate!}^ equal length, or it may 

 have the cilia distributed over certain portions. The cilia are upon the 

 ventral surface in a form known as Stylonychia and in a circle around the 

 blunt end of the trumpet- or bell-shaped body in the forms known as 

 Stentor and Vorticella. These cilia are sometimes varied in size and 

 shape in the different parts of the body. Several types of Infusoria 

 form branching, treelike colonies. 



The Suctoria are attached animals the cilia of which are modified in 

 such a way as to make tentacles of them. These have sucking discs at 



