THE GENERAL SHAPES OF ANIMALS. 187 



scanty, that little beyond negative evidence can be expected 

 from the study of them. 



ml /j/ ua \j 



The like may be said of the Infusoria. These are more or 

 less irregular. In some cases, where the line of movement 

 through the water is tolerably definite and constant, we have 

 a form that is approximately radial — externally at least. 

 But usually, as shown in Figs. 137, 138, 139, there is either 

 an unsymmetrical or an asymmetrical shape. And when one 

 of these creatures is watched under the microscope, the con- 

 gruity of this shape with the incidence of forces is manifest. 

 For the movements are conspicuously varied and indetermi- 

 nate — movements which do not expose any two or more sides 

 of the mass to approximately equal sets of actions.* 



§ 245. Among aggregates of the second order, as among 

 aggregates of the first order, we find that of those possessing 

 any definite shapes the lowest are spherical or spheroidal. 

 Such are some of the Radiolaria, as Collozoum inerme. These 

 bodies which float passively in the sea, and present in turn 

 all their sides to the same influences, have their parts dis- 

 posed with approximate regularity round a centre — approxi- 

 mate, because in the absence of locomotion a slight irregu- 

 larity of growth, almost certain to take place, may cause a 

 fixed attitude and a resulting deviation from spherical sym- 

 metry. The best cases in illustration of the truth here 

 named, are furnished by rotating and locomotive organisms 

 respecting which there is a dispute whether they are animal 

 or vegetal — the Volvocinece. These, already instanced under 



* A verifying comment on this paragraph runs as follows : — " In the 

 Hypotricha Infusoria, which creep over solid surfaces, there is a differen- 

 tiation between ventral and dorsal surface and an approach to bilateral sym- 

 metry. The ventral surface is provided with movable cilia, the dorsal with 

 immobile setae." 



