20 THE ANCESTRY OF VERTEBRATES 



Heymans and Van DER Stricht (1898) in studying the 

 ontogeny of the spinal nerves could not detect spinal ganglia. 

 This is indeed a difficulty from the point of view of my theory 

 which I will not try at all to diminish by any far-fetched 

 hypothesis. The only thing we can do is to assume that 

 the spinal ganglia have fused with the medullary tube or 

 have disappeared in the same way as all the sense-organs 

 and the cerebral ganglia have been lost by Amphicxus 

 which, as we will see, exhibits, in addition to its primitive 

 features, many peculiarities which point to degeneration. 



No stomodaeum in Chordates. — It is in accordance with 

 my theory that in lower Vertebrates, in Amphioxus, Asci- 

 dian larvae and Appendicularia nothing like the ectodermal 

 stomodaeum of the Zygoneura or Protostomia is found. 

 The whole oesophagus is entodermal and the cardia is some- 

 thing quite different from the cardiac pore of Annelids. The 

 mouth, breaking through only very late in embryonic develop- 

 ment, leads directly into the entodermal branchial basket. 



Taste in Chordates. — Now if our supposition is right that 

 the stomodaeum was one of the first sense-organs, that of 

 the taste, we must conclude that the Chordates would have 

 lost this important sense, if they have not acquired a sub- 

 stitute. Probably the neural gland of Tunicates may be 

 explained in this way. This peculiar organ, opening by the 

 ciliated funnel in the roof of the anterior part of the bran- 

 chial basket, originates from the central nervous system, as 

 was pointed out by KOWALEWSKY (1866) for the Ascidians 

 and by myself for the Appendicularia (1912). Evidently a 

 new communication between the former stomodaeum and 

 the gut was established after the old one had atrophied, 

 originating as a diverticulum from the so-called brain-vesicle, 

 afterwards separating from it and thus giving rise to a little 

 ciliated funnel-shaped sense-organ of ectodermal origin in the 

 entodermal gut-wall. It remains in close connection with 

 the cerebral vesicle, the swollen anterior end of the medullary 

 tube, the former stomodaeum. 



In Amphioxus and the Craniata we see arise an ectodermal 

 involution, the oral cavity, into which in Craniata the end- 

 buds — in fishes still widely distributed in the mouth, the 

 branchial cavities and the outer surface of the head, in 

 some even over almost the whole surface of the body — 

 gradually concentrate to form the taste-buds which in higher 

 Craniates are chiefly confined to the epithelium of the tongue 



