Chap. 21 THE PROTOZOANS 443 



plasm flows out of the chamber, spreads over the shell it has made and secretes 

 another shell, a second chamber. Delicate pseudopodia extend through pores 

 in the shell as well as through the main opening. Adult foraminiferans are 

 dimorphic, that is, some individuals divide asexually into many new ones; 

 others divide into flagellate gametes (sex cells). The gametes fuse in pairs and 

 produce individuals that divide asexually when full grown. This alternation of 

 sexual and asexual phases suggests the "alternation of generations" of the 

 coelenterates (Chap. 24). 



The "white cliffs of Dover," England, and the chalk-beds 1 000 feet or more 

 deep of Mississippi and Georgia are made of foraminiferan shells that once 

 dropped downward through deep seas that flooded these lands. Foraminif- 

 erans live in surface waters and Globigerina is one of the commonest of 

 them. These animals are constantly dying and their shells form the "globigerina 

 ooze" that covers some 40 million square miles of ocean floor. 



Radiolarians are among the most beautiful objects in nature. They are a vast 

 array of animals with clear glassy skeletons, radiating needles and latticed 

 spheres of silica fashioned like delicate crystal toys. The protoplasm that is 

 foamy with vacuoles, holds fat drops, oil spheres, and red, yellow, and brown 

 pigment granules. Many of them contain "yellow cells," very minute proto- 

 zoans that live within them. 



Radiolarians are exclusively marine, living chiefly in surface waters; some 

 species have been found in samples taken at depths of over three miles. Their 

 skeletons fall upon the sea bottom in more perfect shape than those of foram- 

 iniferans of the same size because the silica is so resistant to the corrosive 

 effects of the sea water. Probably for the same reason, radiolarians are among 

 the oldest and most perfect fossils known. Many of the old patterns are almost 

 identical with those of present-day species although their microscopic sculp- 

 turing must have been in the making long ages before multicellular animals 

 appeared. 



Parasitic Sarcodina 



Parasitic amebas occur in many vertebrates, in man, and in such dis- 

 tantly related invertebrates as hydras, leeches, and cockroaches. Practically 

 all of them inhabit the alimentary canal and all enter an encysted stage at 

 one time or another in their life history. It is then that they pass out of the 

 body of the host and are freely distributed. Endameba histolytica, the cause 

 of amebic dysentery, lives within the human intestine (colon). In its encysted 

 stage, it is transmitted from one person to another in drinking water and by 

 flies and food. Endameba gingivalis is a common parasite in the human mouth 

 where it lives near the base of the teeth. The colons of cockroaches often 

 contain numbers of Endameba blattae that ingest bacteria from the in- 

 testinal content. 



