84 



GENERAL PHYSIOLOGY 



In the contents of many cells there occurs a fourth group of 

 solid elements, which either do not take part at all in the life- 

 process of the cell, or have ceased to do so. These are indigestible 

 bodies which are taken in occasionally, such as sand-grcans (Fig. 

 26), which are met with in many Amaibce, the indigestible residue 

 of food-stuffs, such as shells, skeletons, and the capsules of food-organ- 

 isms, and excretory substances, which remain for some time in the 

 cell-body as useless by-products or as end-products of metabolism, 

 to be excreted later. 



Finally, among the solid elements of the protoplasm in certain 

 cells, especially in aquatic animals, there occur not rarely symbiotic 

 or parasitic unicellular organisms which strictly do not belong to 

 the protoplasm of the cell in question, but in individual cases play 

 an important role in the life of their host. Among such .symbiotic 

 organisms are especially many algse, the Zooxanthellce and the 



Fig. 26. — ^»Me&«-cell containing in Its 

 protoplasm one diatom shell and two 

 sand-grains. 



Fig. 27. — Paramecium bursai-ia, aciliate- 

 infusorian cell, ttie esoplasm of 

 which is filled with small parasitic 

 alga-cells (ZoochloreUo;). 



Zoochlorellce, the nature of which as independent organisms has 

 been for a long time in dispute. They occur abundantly in 

 the cells of lower animals, particularly in many Infusoria and 

 Badiolaria, to which by the activity of their chlorophjdl-bodies 

 they furnish oxygen, so that as regards respiration their hosts 

 are largely independent of the oxygen of the medium in which 

 they live (Fig. 27). 



We shall not enumerate exhaustively the solid components that 

 are to be met with in cells. Such a list would fill many pages. It 

 is only important here to understand how different in nature are 

 the various solid constituents of protoplasm that may occur in in- 

 dividual cells, and how unjustified is the idea of the unitary character 

 of protoplasm. We will now leave the solid elements, and turn to 

 the consideration of the homogeneous ground-substance. 



