120 THE SHAPES AND SIZES OF ANIMALS 



ever possible to speak of a distinct shape, for 

 when perfectly at rest they all tend to withdraw 

 their processes, and to assume more or less 

 definitely a spherical form. This spherical shape 

 is very characteristic either of the normal condition 

 or of the resting state of a large number of 

 Protozoa, and deserves further notice. When 

 widely departed from, such departure is commonly 

 associated with induration of part of the surface 

 either as a mere thickening of the ectosarc, or in 

 the form of a definite shell ; a condition which is 

 clearly a more modified one, and usually involves 

 and is associated with further differentiation, such 

 as the presence of a definite mouth, and usually of 

 distinct oral and aboral ends to the body. 



Concerning the spherical forms, it may be noted 

 that they are all aquatic and free swimming : 

 aquatic, because in the case of animals with soft 

 body-substance, it is only in water that the body of 

 the animal can be sufficiently supported on all sides 

 to enable the spherical shape to he retained : free 

 swimming, because a habit of crawling leads, as we 

 shall see directly, to definite modification of external 

 shape, involving a distinction between dorsal and 

 ventral surfaces, and almost invariably a further 

 distinction between anterior and posterior ends. In 

 the typically spherical animal, all parts of the 

 surface are equidistant from the centre, and are 

 alike in all respects ; and such an animal will float 

 suspended in the water with any part of the surface 

 uppermost, or any part undermost. 



There is much reason for thinking that the 



