OX THE AXL'EXTRAL FORM OF THE CIWUDATA. 271 



tliein, and thus the typical mode of respiration of the Chordata would be 

 established. 



Phylogeny of the Chordata. It may be convenient to shew in 

 a definite way the bearing of the above speculations on the phylogeny 

 of the Chordata. For this purpose, I have drawn up the subjoined 

 table, which exhibits what I believe to be the relationships of the 

 existing groups of the Chordata. Such a table cannot of course be 

 constructed from embryological data alone, and it. does not fall within 

 the scope of this work to defend its parts in detail. 



MAMMALIA SATHOPSIDA 



PROTO-AMNIOTA AMPHIBIA 



I . 1 



TELEOSTKI PBOTO-PENTADACTYLOIDEI 



GANOIDEI DIPNOI 



i 1 



I'KOTO (IANOIDKI 



HoLOCKPHALI 



ELASMOBHANCHII 

 I'UOTO (iNATUOSTO.MATA 



i 



Oyclo&tomata PROTO-VEKTEBRATA 



Ccpha lochorda ruoTOC 1 1 ( > K DATA Vruchonla 



In the above table the names printed in large capitals are hypothetical groups. 

 The other groups are all in existence at the present day, but those printed in Italics are 

 probably degenerate. 



The ancestral forms of the Chordata, which may be called the 

 Protochordata, must be supposed to have had (1) a notochord as 

 their sole axial skeleton, (2) a ventral mouth, surrounded by suctorial 

 structures, and (3) very numerous gill-slits. Two degenerate off- 

 shoots of this stock still persist in Amphioxus (Cephalochorda), and 

 the Ascidians (Urochorda). 



The direct descendants of the ancestral Chordata, were pro- 

 bably a group which may be called the Proto-vertebrata, of which 

 there is "no persisting representative. In this group, imperfect 

 neural arches were probably present ; and a ventral suctorial mouth 

 without a mandible and maxilke was still persistent. The branchial 

 clefts had, however, become reduced in number, and were provided 

 with gill-folds ; and a secondary head (vide p. 200), with brain and 

 organs of sense like those of the higher Vertebrata, had become 

 formed. 



The Cyclostomata are probably a degenerate offshoot of this 

 group. 



With the development of the branchial bars, and the conversion of 

 the maud ibular bar into the skeleton of the jaws, we come to the Proto- 

 gnathostomata. The nearest living representatives of this group are 

 the Elasmobranchii, which still retain in the adult state the ventrally 



