148 ANIMAL BIOLOGY 



gregation of foreign particles. One large group of these 

 organisms, the Foraminifera, is abundantly represented 

 in the sea, in certain parts of which extensive deposits are 

 formed by the accumulation of their minute shells. Chalk 

 is a deposit which is mainly formed of the shells of these 

 animals. Another very large group, the Radiolaria, is 

 confined to the sea. Many species have beautiful silicious 

 skeletons and in some parts of the ocean there are exten- 

 sive deposits formed from the remains of these animals. 



FIG. 121. Shells of Foraminifera from the bottom of the Indian Ocean. 



The Sporozoa, which, as the name implies, are charac- 

 teristically spore-producing organisms, are all parasitic 

 in the bodies of animals. One large division, the Grega- 

 rines, are parasitic in invertebrate animals, where their 

 favorite situation is in the alimentary canal. Another 

 group, the Coccidea, infest both vertebrates and inverte- 

 brates. The Haemosporidia are blood parasites and in- 

 clude the common malaria parasites (Plasmodium) which 

 have already been mentioned in treating of the mosquito. 

 The organism that causes Texas fever in cattle and which 



