I 3 4 THE SHAPES AND SIZES OF ANIMALS 



such forms as Anonymus, in which the body is 

 greatly flattened and almost circular in outline, and 

 in which the mouth is almost in the centre of the 

 ventral or oral surface, while a row of eye spots 

 occurs all round the edge of the animal, though 

 more thickly set at the two extremities. Starting 

 from such a form as Anonymus, which may be 

 compared to a very flat jelly-fish, like Aurelia, 

 which instead of swimming freely in the water, has 

 taken to crawling about on the sea-bottom mouth 

 downwards, we find two diverging series in both 

 of which the body gradually becomes more and 

 more elongated and vermiform in shape, while the 

 sense organs tend to become concentrated at the 

 anterior end. In one series, that of the Turbel- 

 laria Acotylea, the mouth gradually moves backwards 

 as the shape of the body becomes more markedly 

 oval and elongated ; in the other series, the Tur- 

 bellaria Cotylea, which receive their name from the 

 presence of a muscular sucker on the ventral sur- 

 face, the mouth, starting as in the Acotylea from a 

 central position, gradually shifts further and further 

 forwards until it ultimately, in the genus Prosthio- 

 stomum, becomes placed quite at the anterior end of 

 the body. 



There is little room for doubt that, just as among 

 epithelial cells the spherical form is the primitive 

 one from which both the columnar and the squamous 

 forms have been derived, so also in the series of 

 Turbellarian worms, those with the body approxi- 

 mately circular in outline and with a centrally 

 placed mouth are really the primitive ones from 



