134 



ANIMALS 



tents, so far as external forces are concerned, an inert body and 

 has lost its animal character completely. When expanded and 

 moving, on the other hand, the animal character reappears and 

 at the same time the form of the animal becomes largely or 

 completely bilaterally symmetrical. 



320. The free swimming larvae of the Echinoderms are per- 

 fectly bilateral, but when they assume the less active or fixed 

 life habit of the adult, they become radial in symmetry. This 

 change involves a radical metamorphosis. In the case of some 



Fig. 64. — The bilaterally symmetrical free swimming larva of an Echinoderm. 



(From Ziegler's models.) 



of the Holothuria a second change occurs, in which the radial 

 symmetry is largely superseded by a secondary bilateral sym- 

 metry. This is brought about by the habit of the animal of 

 assuming a horizontal instead of a vertical position of the 

 principal axis. 



321. The adult ascidians and barnacles also show a strong 

 tendency toward radial symmetry, although the active larvae 

 are bilateral. 



322. A striking example of a different type is offered by the 

 ''flat fishes," such as the flounder and sole. The young of 

 these fishes have the ordinary type of bilateral symmetry, and 

 in swimming they also assume the erect position characteristic 



