GENERAL ZOOLOGY 



Fig. 8.6. Representative shelled rhizopods. .4, 

 Arcella, and B, Difflu^ia, order Lobosa. C, Actinophrys, 

 order Heliozoa. /), Clohi^erma, order Foraminiferg. 

 (/), redrawn from W. C. Williamson, 1858, Mono- 

 graph on the Recent Foraminifera of Great Britain.) 



series of cell division by binary fission, although more complicated phenomena, 

 such as encystment and sexual reproduction, have been described. Present in- 

 dications are that Amoeba pwteiis, for example, reproduces only by binary fission 

 (Fig. 8.5), with subsequent growth of the daughter cells to full size, continuing 

 in the active state without syngamy or encystment. Amoebas may become 

 smaller through starvation, or, as in some larger species, multinucleate forms 

 may be produced by the failure of the cytosome to divide following nuclear 

 division. The large fresh-water amoeba, Pelomyxa carolinensis, contains hundreds 

 of nuclei produced in this way. At the time of cell division, the cytosome 

 divides, distributing the nuclei between the resultant daughter individuals. In 

 some of the other amoeboid forms, more complicated life cycles, with budding 

 and encystment, have been discovered. Some of these cycles include flagellated 

 stages, and in others, gametes and syngamy are known. 



Other Sarcodina. The genus Amoeba, with many other free-living forms 

 which it resembles, is placed in a subdivision of the Sarcodina known as the 

 Rhizopoda. Other rhizopods, such as the genera Arcella and Difflugia, possess shells 

 with a single opening from which the pseudopodia extend (Fig. 8.6). In the 

 Foraminifera, there is a shell composed of calcium carbonate, chitin, silica, or 

 other materials, and the pseudopodia extend through numerous openings. With 

 few exceptions, the Foraminifera are marine, living near the surface of the sea 

 as well as on the bottom. The shells of dead foraminiferans make up a large 

 part of the silt that covers the ocean fioor, in regions such as the deeper 

 portions of the North Atlantic. In chalk formations in various parts of the 

 world, some of them hundreds of feet thick, these minute shells make up as 

 much as 70 per cent of the deposits. Another subdivision, the Actinopoda, 

 includes the "sun animalcules," such as Actinophrys sol (Fig. 8.6), and the 

 Radiolaria, which are notable for their siliceous skeletons of great beauty and 

 variety (Fig. 8.7). In some of the very ancient sedimentary rocks occur 



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