THE DIGESTIVE SYSTEM 



299 



mouth or that it is homologous with the annelid mouth lacks sufficient 

 evidence to convince most zoologists that it is more than an interesting 

 speculation. 



According to Von Kupffer, the hypophysis of vertebrates represents a 

 paleostoma which functioned as a mouth in prechordates, following their 

 abandonment of the blastoporic mouth. In support of this assumption, 

 he points out that the definitive mouth of vertebrates arises late in onto- 

 genesis in such relation to the series of gill-slits that it might have been 

 formed from a pair of coalesced gill-sHts; that the presence of a pre-oral 



HYPOPHYSIAL 

 DUCT 



A.BDELLOSTOMA. pharynx/ 



HYPOBRANCHIAL MUSCLE 



(RESPIRATORY TUBE 



HYPOPHYSIAL ^ 



DUCT ^=-^ 



NOTOCHORO 



I ST GILL APERTURE 

 B. PETROMYZON. 



HYPOBRANCHIAL MUSCLE 



Fig. 250. — Diagrams of median longitudinal sections of the heads of Bdellostoma and 

 Petromyzon, showing the relations of the hypophysial ducts in the two forms. In the 

 former the hypophysial duct opens posteriorly into the pharynx, suggesting the possibil- 

 ity that it may once have served as a mouth (paleostoma). In petromyzon the hypo- 

 physis fails to open into the pharynx and is converted into a pipette-like organ into 

 which the olfactory pits open. On the basis of this difference cyclostomes are divided 

 into two sub-classes, Hyperotreta and Hyperoartia. 



gut in vertebrate embryos suggests that the alimentary canal formerly 

 extended anterior to the present mouth; and, finally, that in the Myxinoids 

 and the embryonic sturgeon the hypophysis actually opens into the 

 pharynx, and like the mouth of urochordate larvae, has a dorsal external 

 opening. Kupffer, however, does not waste time guessing as to the cause 

 of this substitution of a new mouth for an old one. It is possible that 

 the development and enlargement of the pre-oral lobe was a factor 

 in effecting the displacement of the mouth from a dorsal to a ventral 

 position. Diagrams showing the position of the four mouths mentioned 

 in this discussion are shown in Fig. 249. The objection to the idea that 

 there have been a series of mouths in the course of animal phylogenesis, 

 on the ground that the chances are against the appearance of more than 

 one ingestive opening into the enteron, loses much of its weight in view 



