i8 



THE ORIGIN OF VERTEBRATES 



Hy. 



we see that Dohrn's original view of the comparison of the vertebrate 

 and the annelid led him to the conception that the vertebrate mouth 

 was formed by the coalescence of a pair of gill-slits, and that the 

 original mouth was situated somewhere on the dorsal surface and 

 opened into the gut by way of the infundibulum and the tube of the 

 hypophysis. This, also, was Cunningham's view as far as the tube 

 of the hypophysis was concerned. Beard, in 1888, holding the view 

 that the vertebrates were derived from annelids which had lost their 

 supra- oesophageal ganglia, and that, therefore, there was no question 

 of an oesophageal tube piercing the central nervous system of the 

 vertebrate, explained the close connection of the infundibulum with 

 the hypophysis by the comparison of the tube of the hypophysis with 



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Fig. 125.— Diagram to show the Meeting op the Four Tubes in such a 



Vertebrate as the Lamprey. 



Nc, neural canal with its infundibular termination ; Nch., notochord ; Al., alimentary 

 canal with its anterior diverticulum ; Hy., hypophysial or nasal tube ; Or., oral 

 chamber closed by septum. 



the annelidan mouth, so that the infundibular or so-called nervous 

 portion was a special nervous innervation for the original throat, 

 just as Kleinenberg had shown to be the case in many annelids. 

 Beard therefore called this opening of the hypophysial tube the old 

 mouth, or palaeostoma. Eecently, in 1893, Kupffer has also put 

 forward the view that the hypophysial opening is the paheostoma, 

 basing this view largely upon his observations on Ammoccetes and 

 Acipenser. 



As is seen in Fig. 125, the position of this paheostoma is a very 

 suggestive one. At this single point in Ammoccetes, four separate 

 tubes terminate ; here is the end of the notochordal tube, the termina- 

 tion of the infundibulum, the blind end of the nasal tube or tube 



