/. RHIZOPODA: FORAMINIFEBA. 



197 



In the fresh-water forms the shell is built of a chitinuoiis 

 organic substance which may be strengthened by silica or the 

 incorporation of foreign particles. The more tyi^ical members, 

 exclusively marine, have almost invariably calcareous shells which, 

 when dissolved by acid, leave but the slightest trace of organic 

 matter. The presence of minute jiores in the shell is of syste- 

 matic importance, the group of Perforata (fig. 117) being char- 

 acterized by them. 



The animal portions form a more or less complete cast of the 

 inside of the shell (fig. 120), and in the polythalamate forms con- 

 sist of as many pieces as there are chambers in the shell, connected 



Fig. 129.— Protoplasm of Globigerina 

 after solution of the shell. )i, nucleus. 



Fig. 130. — Young Miliola with several 

 nuclei. (From Lang.) 



together liy plasma bridges passing through the foramina of the 

 partitions. In the protoplasm there is a large nucleus (figs. 128, 

 l.'jO. n), M'hich in some cases is early replaced by a daughter gen- 

 eration of small nuclei. Contractile vacuoles usually occur only 

 in the fresh-water forms. Tlie pseudopodia project through the 

 chief opo]iing of the shell and in the Perforata probably througli 

 the pores in the shell wall. They are rarely finger-like (fig. 128); 

 usually they are thread-like, branched, richly granular aud 

 anastomosing, and hence favoralilc ol)jects for the study of the 

 streaming of protoplasm. 



Reproduction is generally accf>mplished by fission, but presents 

 many variations. Only rarely do both animal and shell divide; 

 frequently the protoplasm protrudes from the mouth of the shell, 

 a new shell is formed on the outgrowth and the protoplasm then 

 divides, one of the resulting individuals retaining the old shell. In 

 the marine Polythalamia the following process is general: The 



