166 UNICELLULAR ANIMALS. 



ditions of superficial layers, as in the case with growth by accre- 

 tion (inorganic bodies, e.g., crystals). Under favorable condi- 

 tion of nutrition this process exceeds the destructive process so 

 that the body increases in size up to a limit, at which fission 

 takes place. AVhat determines this limit is unknown, but the 

 cause is j)erhaps in some way connected with the geometrical 

 principle that the volume of the cell increases as the cube of its 

 diameter, whereas the surface, by wliich it absorbs nutriment, 

 and otherwise comes into relation with the outside world, in- 

 creases only as the square of the diameter. 'No great increase 

 in size, therefore, is possible without destroying the normal equi- 

 librium of the cell and hence the periodic reduction of size by 

 division. This principle is, however, too general to be of much 

 value. Different species of Amceha differ in size-limit, and the 

 immediate cause lies in some subtle relation between organism 

 and environment that cannot at present be made out. It is not 

 known whether or not the Amoeba ever dies of old age. 



These "fundamental physiological properties" of proto- 

 plasm lie at the basis of all physiology, and will be found ap- 

 plicable to all forms of life whether vegetal or animal. 



Related Forms. Amoeba is a representative of a very extensive class of 

 Protozoa known as Rhizojjoda^ all characterized by the power to form 

 pseudopodia, and agreeing with Amoeba in many other respects. One of 

 the commonest fresh- w^ater forms is the genus A7'ceUa (Fig. 86, C), which 

 even in the active phase is surrounded by a brown horny membrane 

 ("shell") perforated by a large rounded opening through w^hich pseudo- 

 podia are protruded. DiMugia (Fig. 86, B), also a common fresh-water 

 form, builds about itself a beautiful vase-shaped or retort-shaped shell 

 composed of sand-grains, or even, in some cases, of diatom-shells. In 

 ActinopJirys^ or the "sun-animalcule" (Fig. 86, A), the pseudopodia are 

 stiff needle-shaped processes radiating in every direction. 



Among the marine forms two groups (orders) are of especial interest 

 and importance ; viz., the Foraminifera, which secrete a calcareous shell 

 perforated by numerous pores, and the Radiolaria^ which have a siliceous 

 shell. Many of these forms float at the surface of the water, and their 

 cast-off shells have in former times accumulated at the bottom in such 

 enormous quantities as to form beds of chalk in the case of Foraminifera, 

 while the remains of Radiolaria have made important contributions to the 

 formation of siliceous rocks. 



