214 



EVOLUTION AND GENETICS 



Aquatic Adaptation. Purely aquatic organisms are safe from 

 dessication, consequently they have no need of a moisture con- 

 serving integument. They are buoyed up by the water in which 

 they live so effectively that they have no need of rigid structure. 

 The result is that the most delicate organisms are aquatic. Jelly- 

 fishes are made up mostly of water (Fig. 124). Their beautiful, 

 filmy bodies, if removed from the water, fall into a shapeless heap 

 and quicldy dry into a small organic residue. 



Benthos. Some of the aquatic animals remain on a solid sub- 

 stratum, and are either attached to immersed objects or move 

 from place to place over the bottom (Fig. 125). Some rigidity is 



Fig. 125. — Sea anemones. (From Hegner, after Coleman.) 



obviously necessary in these animals as protection, not only 

 against their enemies, but also against the motion of the water in 

 which they live. It is also an advantage for the sedentary forms 

 to be able to reach out in all directions, since their food must come 

 within reach instead of being sought. Consequently radial sym- 

 metry is a common character. Radially symmetrical animals are 

 made up of similar parts arranged about a common center, in con- 

 trast to the more common bilaterally symmetrical forms, which are 

 made up of similar halves flanking the longitudinal axis. Regard- 

 less of their powers of motion, these bottom forms are called the 

 benthos. The shallow seas are rich in benthonic forms, such as the 

 sponges, sea anemones, corals, barnacles and many others. ' 



