Oy THE ANCESTRAL FORM OF THE CHORD AT A. 271 



them, and thus the typical mode of respiration of the Ohordata 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 liave drawn np 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. Sauropsida 



I'HOTO-AMMOTA AMPHIBIA 



Teleostei I'UOTO-l'E.NTADACTYLOlDEI 

 Ganoidei i Dipnoi 



PROTO-GANOinEI 



j HOLOCEPHAL.1 



i Elasmobkancuii 



PUOTO GNATIIOSTOMATA 



I 

 _ . -I 



Cyclostovuita PROTO- VERTEBU AT A 



1 



Cephalochorda PROTOCHORDATA Urochorda 



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

 The other groups are all in existence at the present day, but those printed in Itahcs 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 maxillae was still persistent. The branchial 

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

 with gill-folds ; and a secondary head (vide p. 260), with brain and 

 organs of sense like those of the higher Yertebrata, 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 mandibular 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 



