Beaches 21 



waves; radial in symmetry and conical, so that waves easily slide 

 over it; and adapted to catching food from water by spreading 

 tiny, appendicular nets. A barnacle has long been used as an 

 example of "retrogressive metamorphosis." During successive molts 

 it changes from a free-swimming, appreciative (in that it has 

 rather elaborate sense organs) , and active animal to one that is 

 unappreciative, sessile, and enduring rather than thoughtful. Such 

 an animal as a barnacle gives no promise of producing a land 

 animal in the future. Its habits and inheritance make it psychologi- 

 cally unprogressive. The same is true of many other typical beach 

 animals. 



Sessile Uttoral animals have not become terrestrial (Harms, 1929; 

 Pearse, 1922), yet there are a few of the active animals, such as 

 certain crustaceans and snails, that apparently have progressed 

 directly across sea beaches to land. At Tortugas the ability of a 

 variety of littoral animals to live in the atmosphere and in various 

 dilutions of sea water was tested. Most of the animals lived longer 

 in the air than in the water. This suggests that they showed little 

 tendency to migrate to land by first becoming adjusted to fresh 

 water (Pearse, 1929) . As has been stated, certain land crustaceans 

 go directly to the ocean to breed each year. Cannon (1923) studied 

 the development of the eggs of a land crab, Cardisoma armatum 

 Herklots, in various solutions. He found that all eggs hatched in 

 sea water; a few hatched in half fresh and half sea water; and all 

 died in fresh water. Algae high up on beaches are more resistant 

 to desiccation than those which live in lower zones and grow more 

 slowly; snails at higher levels show greater negative geotropism and 

 positive phototropism than those below (Colman, 1933). There 

 are many evidences that animals have passed directly, but of course 

 slowly, from ocean to land. 



The little pools which are left between tide marks when the ocean 

 periodically recedes serve as refuges for some marine animals and 



