THE DIGESTIVE SYSTEM 



219 



pre-oral 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. (Fig. 208) 



Whatever view is held of the origin of vertebrates, we must believe that 

 there have been at least two mouths in the course of vertebrate phylogene- 

 sis. The reason for this conclusion is that the original coelenterate mouth 

 becomes the mouth in no vertebrate, while only in cyclostomes, dipnoans, 

 and possibly some amphibians does it become the anus. The coelenterate 



HYPOPHYSIAL 

 DUCT 



A.BDELLOSTOMA. PHARYNX/ 



,1 ST GILL 

 APERTURE 



HYPOBRANCHIAL MUSCLE 



HYPOPHYSIAL ^ 



DUCT ^=-^ 



(RESPIRATORY TUBE 



NOTOCHORD 



I ST GILL APERTURE 



B. PETROMYZON. 



HYPOBRANCHIAL MUSCLE 



Fig. 206. — 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 possi- 

 bility 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. 



mouth becomes the blastopore of chordate embryos. And the blastopore 

 of chordates lies at the posterior end of the body and forms the neurenteric 

 canal, which connects the neural tube with the enteron, while the chordate 

 mouth develops at the anterior end of the enteron. Consequently, it 

 seems indisputable that there have been at least two mouths in the history 

 of vertebrates. 



While, however, all agree that the vertebrate mouth is not the primary 

 animal mouth, and that at least two mouths have successively appeared, 

 some rnorphologists believe that there have been at least three mouths, 

 Delsman (1922), reviving an earlier suggestion of Kowalevsky (1877), 

 claims that " in the ontogeny of vertebrates we see three successive mouths 

 appear in the same succession as they appeared in phylogeny, viz., the 

 blastopore (Urmund), the neuropore (the annelidan mouth), and finally 



