CHAPTER XIV. 



THE GENERAL SHAPES OF ANIMALS. 



244. CERTAIN of the Protozoa are quite indefinite in 

 their shapes, and quite inconstant in those indefinite shapes 

 which they have the relations of their parts are indeter- 

 minate both .in space and time. In one of the simpler 

 Rhizopods, at least during the active stage of its existence, 

 no permanent distinction of inside and outside is established ; 

 and hence there can arise no established correspondence 

 between the shape of the outside and the distribution of 

 environing actions. But when the relation of inner and 

 outer becomes fixed, either over part of the mass or over the 

 whole of it, we have kinds of symmetry that correspond 

 with the habitual incidence of forces. An Amoeba in be- 

 coming encysted, which we may regard as the production 

 in it of a differentiation between superficial parts and central 

 parts, passes from an indefinite, ever-changing form into 

 a spnerical form ; and the order of symmetry which it thus 

 assumes, is in harmony with the average equality of the 

 actions on all its sides. In Di/flugia^ Fig. 134, and still 

 better in Arcclla, we have an indefinitely-radial symmetry 

 occurring where the conditions are different above and below 

 but alike all around. Among the Gregarinida the spherical 

 symmetry and symmetry passing from that into the radial, 

 are such as appear to be congruous with the simple cir- 

 cumstances of these creatures in the intestines of insects. 



