CHAPTEE 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 indeterminate 

 both in space and time. In one of the simpler Khizopods, 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 Amceba in becoming encysted, 

 passes from an indefinite, ever-changing form into a spherical 

 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 Difflugia, Fig. 134, and still better in Arcella, 

 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 sym- 

 metry passing from that into the radial, are such as appear 

 to be congruous with the simple circumstances of these 

 creatures in the intestines of insects. But the relations of 

 these lowest types to their environments are comparatively 

 so indeterminate, and our knowledge of their actions so 

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