296 



COMPARATIVE ANATOMY 



The ectodermal horny teeth of cyclostomes Hke the movable lips and 

 cheeks in mammals, are merely details which do not affect the general 



issue. 



But concerning the question of the homology of the mouth of verte- 

 brates with that of the lower chordates, opinion is divided. Van Wijhe 



OLFACTORY PIT 



^OLFACTORY PIT 



HYPOPHYSIS 



OLFACTORY 



PIT 



HYPOPHYSIS, 



NEOSTOMA' 



lUllllliiiiiiiiiiiiriiim 

 PHARYNX 



iriiiii'i'"! 



jHYPOPHYSIS 



PHARYNX 



NEOSTOMA 



I NEOSTOMA 



AMPHIOXUS. PETROMYZON. 



Fig. 248. — Diagrams illustrating the paleostoma theory. A-D show stages in the 

 development of a pre-oral (hypophysial) opening as in myxinoids and, according to 

 Legros, as in Amphioxus. E-H show stages in the development of the hypophysis in 

 petromyzon, in which the neostoma supplants the paleostoma. In the higher verte- 

 brates the hypophysis is converted into an important endocrinal organ. (Redrawn 

 from Johnston after Legros.) 



(1914) suggested that the mouth of Amphioxus is not homologous 

 with that of vertebrates, but is comparable with the left spiracle of 

 elasmobranchs. 



The foundations of this assumption, however, are weak. The enor- 

 mous enlargement of the larval mouth of amphioxus, as well as its asym- 

 metry are best interpreted as larval adaptations. 



