220 



CHORDATE ANATOMY 



.BLASTOPORE 



NEUROPORE. 



^""^<^^"' 



A. B. C. 



Fig. 207. — A diagram illustrating the way in which, according to Delsman, the blasto- 

 poric mouth of coelenterates is in chordates converted into the neurenteric canal. 

 Delsman homologizes the chordate neural tube with the ectodermal f oregut of annelids. 



NEURAL NOTOCHORD 



BLASTOPORE NEUROPORE TUBE | BLASTOPORE 



ENDODERM 



A.GASTRULA 



GILL POUCHES 



B.AMPHIOXUS EMBRYO 



ENTERON 



MOUTI 



NEUROPORE 



NOTOCHORD 



GILL POUCHES 



C.UROCHORDATE LARVA 



ENDODERM STRAND 



NEUROPORE 



OTIC CAPSULE 



notocho^Jd'^'^nte^ic^canal^^ 



POST-ANAL GUT 



ANUS 



GILL POUCHES 

 DEFINITIVE MOUTH 

 HYPOPHYSIS 



D. VERTEBRATE 



Fig. 208. — Diagrains illustrating the hypothetical phylogenesis of the vertebrate 

 mouth. The primitive animal mouth, the blastopore, is converted in vertebrates either 

 into an anus or a neurenteric canal. The definitive mouth of vertebrates therefore is a 

 secondary mouth. But the relations of the neuropore are such that at one time in the 

 ancestry of chordates this may have served as a mouth and the neural tube as a f oregut. 

 It is also possible that the mouth of urochordates is not homologous with the definitive 

 mouth of vertebrates. The evidence of a paleostoma or hypophysial opening suggests 

 that this may once have been a functional mouth. Thus the definitive mouth may have 

 been the last in a series of four mouths. 



