TAXONOMY, ANATOMY, EMBRYOLOGY 



(squids, octopi, and their allies). Superficially, the cephalopod eye bears 

 a close resemblance to that of the vertebrates, but detailed examination 

 shows that in every part difl^erent materials have been used in different 

 ways. Embryologically, the cephalopod eye develops from the skin, while 

 the vertebrate eye develops from the brain, with the exception of the lens, 

 which is a skin derivative. Yet even the lenses differ fundamentally, for 

 that of the vertebrates is cellular, that of the cephalopods a crystalline 

 secretion of skin cells. Vertebrate and cephalopod eyes are only analogous. 

 Another classical example is provided by the wings which have been 

 developed independently by insects, reptiles, birds, and bats ( Figure 17 ) . 

 All show analogical resemblances because they are adapted to the same 

 function. The insect wing shares nothing but the planing surface with the 

 others, for it is simply a membrane supported by chitinous veins. All of 

 the vertebrate wings are constructed from the typical parts for the fore- 

 limb of a tetrapod (land vertebrate), and to this extent they may be 

 considered homologous. But the several groups of flying vertebrates are 

 not descended from a common ancestor which flew: flight has been de- 

 veloped independently in three diflerent lines of descent. The pterodactyl, 



FiGtTRE 17. Analogical Re- 

 semblances OF THE Wings 

 OF an Insect (A), a Bird 

 (B), AND A Bat (C). In 

 each, a planing surface is 

 formed from completely dif- 

 ferent materials. 



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