178 



PROTOZOA 



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

 algae (Zooxanthellcr), which are also present in other animals. (Thalamophora, 

 actinians, sponges, etc.). They afford an instance of symbiosis, or the living 

 together of different organisms for mutual good. The Radiolaria are exclusively 

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

 Certain species and even large groups (Phaeodaria) occur only at great depths 

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



Order V. Thalamophora (Foraminifera, Reticularia). 



The Foraminifera, though not equalling the Radiolaria in beauty and 

 variety of forms, exceed them in numbers of individuals, and have a great 

 importance in the history 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, which 

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

 through the aperture (fig. 129). Accordingly as the axis connecting 

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



or even coiled in a spiral. The interior of 

 the shell is frequently divided by transverse 

 partitions into numerous chambers (fig. 131). 

 Such many-chambered shells (Polythalamia) 

 are at first small, and consist of one or few 

 chambers, but as the animal grows new 

 chambers are added at the mouth of the 

 shell. Openings (foramina') in the walls 

 connect the adjacent chambers. The spiral 

 shells with many chambers have a striking 

 resemblance to the shells of the Nautilus 



(% 35 2 )- 



In the fresh-water forms the shell is 



built of an organic substance which may 

 be strengthened by silica or the incorpora- 

 tion of foreign particles. The more typical 

 members, exclusively marine, have 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 inside of the shell (fig. 130), 

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

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

 the protoplasm there is a large nucleus (figs. 129, 130, ), which in some 

 cases is early replaced by daughter nuclei. Contractile vacuoles usually 

 occur only in the fresh-water forms. The pseudopodia project through 



FlG. 129. Quadrula sym- 

 metric a (after F. E. Schulze). 

 cv, contractile vacuole; n, nu- 

 cleus; TV, food-body. 



