17S 



PROTOZOA 



Common, if not constant, in the Radiolaria arc the yellow cells, unicellular 

 alga; (ZooxaiilliclUr), which arc also present in other animals. (Thalamophora, 

 actinians, sjionges, etc.)- They afi'ord an instance of symbiosis, or the liyin;:^ 

 together of different organisms for mutual good. The Radiolaria are exclusi^■cly 

 marine. In fair weather they float at the surface, but sink in times of storm. 

 Certain species and eyen large groups (Pha'odaria) occur only at great depths 

 (1500-4000 fathoms); seyeral thousand species known. 



Order V. Thalamophora (Foraminifera, Reticularia). 



The Foraminifera, though not equalling the Radiolaria in beauty and 

 variety of forms, exceed them in numliers of indiyiduals, and haye a great 

 importance in the Itistory of the earth. No other group of animals has 

 had so great a part in the formation of beds of rock. 



The most prominent characteristic is afforded by the shell, ^yhich 

 is closed at one pole, and usually open at the other, the pseudopocha passing 

 through the aperture (Itg. i^q). Accordingly as the axis connecting 

 these poles is altered, the shell becomes disc-like, spherical, flask formed 



or eyen coiled in a spiral. The interior of 

 the shell is frequently diyided by traiisyerse 

 partitions into numerous chambers (fig. 131). 

 Such many-chambered shells (Polythalamia) 

 are at first small, and consist of oiae or few 

 chambers, but as the animal gro^ys new 

 chambers are added at tlie mouth of the 

 shell. Openings ij'oramina) in the \yalls 

 connect the adjaceiit chambers. The spiral 

 shells witli many chambers ha\-e a striking 

 resemblance to the shells of the Nautilus 

 ('ig- 352)- 



In the fresh-water forms the shell is 

 built of an organic substance wliich may 

 be strengthened by silica or the incorpora- 

 tion of foreign particles. The more typical 

 members, exclusiyely marine, haye cal- 

 careous shells with but the slighest trace of organic matter. The presence 

 of minute pores in the shell is of systematic importance, the group of 

 Perforata (fig. 118) being characterized by them. 



The animal portions form a cast of the iitside of the shell (fig. 130), 

 and consist of as many pieces as there are chambers in the shell, connccteil 

 by plasma bridges passing through the foramina of the partitions. In 

 the protoplasm there is a large nucleus (figs. 12Q, 130, it), \yhich in some 

 cases is eariy replaced by daughter nuclei. Contractile vacuoles usually 

 occur only in the fresh-^yater forms. The pseudopotlia project through 



Fig. 121). — OuaJnila syiii- 

 vtctrica (after I''. K. Schulze) 

 cv, contractile vacuole; }i, nu- 

 cleus; A', food-body. 



