THE SHAPES AND SIZES OF ANIMALS 135 



which the more modified Cotylea and Acotylea 

 have alike sprung. It is very significant that these 

 more primitive Turbellarians should present many 

 points of affinity with the radiate animals, such as 

 Ccelenterates, not merely in general shape but in 

 the position of the mouth and central gastral 

 chamber, the radial arrangement of the diverticula 

 of the gastral chamber by which nutriment is dis- 

 tributed to all parts of the animal, the disposition 

 of the sense organs all round the margin of the 

 animal, and the position and relations of the ner- 

 vous system and reproductive organs. These 

 resemblances are too close and too fundamental to 

 be accidental, and they lend much support to the 

 view hinted at above, that bilateral animals are 

 descended from radiate ancestors, that bilateral 

 symmetry is something additional to and imposed 

 on the radiate symmetry, and that this further 

 modification is a direct consequence of the animals 

 having exchanged their pelagic free-swimming 

 habits for crawling ones ; a change that would at 

 once lead to the establishment of a difference, both 

 structural and physiological, between the ventral 

 or oral surface along which locomotion is effected, 

 and the opposite or dorsal surface ; while the 

 further differentiation between anterior and posterior 

 ends would very soon follow as a necessary con- 

 sequence of this same crawling habit. It is very 

 possible also that the Turbellarians are themselves 

 the simplest group in which the crawling habit has 

 been acquired, and are directly descended from 

 Ccelenterate ancestors, a view that finds favour 



