TAXONOMY, ANATOMY, EMBRYOLOGY 



mav have been a primitive character of the vertebrates, and that the 

 pineal gland of most extant vertebrates is in fact a vestigial eyestalk. But 

 if so, it mav well have an important new function, for it is commonly re- 

 garded as an endocrine gland, although the evidence is incomplete. 



EVIDENCE FROM EMBRYOLOGY 



Comparative embryology is a specialized branch of anatomy which fur- 

 nishes evidence for evolution reo;arded bv Darwin as "second to none in 

 importance." Ernst Haeckel brought this field into prominence in the 

 immediate post-Darwinian period with his Biogenetic Law, which states 

 that "Ontogeny recapitulates phylogeny." He believed that embryonic 

 stages corresponded to ancestral adults, and hence provided direct evi- 

 dence of lines of descent. Recapitulations do occur, but not as Haeckel 

 thought, for resemblances are chiefly between embrvos, not embryos and 

 adults, and embryos, too, have adaptive problems. 



A striking example occurs among crustaceans. A series of six larval types 

 (Figure 19) strongly resemble adults in a sequence from primitive to 

 advanced. The larvae pass through these stages at successive molts, with 

 larvae of primitive crustaceans stopping early in the series and advanced 

 crustaceans going through most of the stages, as indicated in the chart. 

 Failure of the Cv'pris larva to appear in the development of higher crusta- 



nouplius 



Qdult 

 Cypris 



naupiius 



stage of 



Cypns 



oduit 



bornode 



cypfis 

 stage of 

 bornocie 



rouplius 

 stoqe of 



bornode 



MyS(S 



2oeo 

 sfaqe of 

 Mysts 



pfoiozoeo 

 sfoge of 



Mys'S 



nouplius 

 stoge of 



MySiS 



oduit 

 Penoeus 



nouplius 

 s'oge of 



Penoeus 



oduii 

 lobster 



megolops 



stoge of 



cnsb 



^noupiius^ 

 stoge of 

 . crob/ 



Figure 19. Developmental Stages of Chustaceaxs. At the top arc a series of larval 

 types, se\eral of which resemble adults of corresponding groups of crustaceans. The 

 sequences of squares at the bottom indicate the developmental histories of the major 

 crustacean tvpes. Stages in o\ als are completed in the egg, but others are free-swim- 

 ming larvae. (From MacGinitie and MacGinitie, "Natural History of Marine Animals," 

 McGraw-Hill Book Co., Inc., New York, N.Y., 1949.) 



45 



