ZOOGENESIS 



Practically all insects in the adult stage are capable 

 of active locomotion either by flying or by running. 

 Most adult insects therefore have exceedingly good 

 eyes, though the structure of the insect eye is very 

 different from the structure of the eyes of the verte- 

 brates. But many sluggish insects incapable of flight 

 have eyes of very limited capacity and some, chiefly 

 cave insects, have no eyes at all. Active crustaceans, 

 such as the land-crabs, crabs (fig. 19, p. 47), hermit- 

 crabs (fig. xy, p. 47) and lobsters, have good eyes v^hich 

 are patterned after the insect model. The inactive 

 kinds have much reduced, simple, or more or less rudi- 

 mentary eyes, or else are v^holly blind. 



Among the mollusks the cuttle-fish and squid (fig. 

 45, p. 97), w^hich are powerful and active creatures, 

 many of them capable of swimming with great speed 

 and a few even of flying like the flying-fishes, all have 

 large and perfect eyes. So do the octopuses which are 

 very active crawlers and also, for short distances, 

 swimmers of no mean ability. Superficially the eyes 

 of the squid and of the octopus appear quite the same 

 as the eyes of fish and other vertebrates, but their 

 development is very different. 



The eyes of the vertebrates, arthropods and mol- 

 lusks, all equally efficient for the purpose which they 

 serve, are wholly independent structures. Each of 

 these types of animals has a type of eye wholly con- 

 fined to it and differing in origin from the other two. 

 But regardless of their origin all three types of per- 

 fect visual eyes are the same in function and in 

 purpose. 



[79] 



