ON THE ANCESTRAL FORM OF THE CHORDATA. 327 



of the alimentary tract of many Chostopods. This part of the alimentary 

 tract was probably provided with paired cascal pouches with their blind ends 

 in contiguity with the skin. 



Perforations placing these pouches in communication with the exterior 

 must be supposed to have been formed ; and the existence of openings into 

 the alimentary tract at the end of the tentacles of many Actiniae and of the 

 hepatic diverticula of some nudibranchiate Molluscs (Eolis, &C. 1 ) shews that 

 such perforations may easily be made. On the formation of such perfora- 

 tions the water taken in at the mouth would pass out by them ; and the 

 respiration would be localized in the walls of the pouches leading to them, 

 and thus the typical mode of respiration of the Chordata would be esta- 

 blished. 



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 SAUROPSIDA 



i _. r _ _ j 



PROTO-AMNIOTA AMPHIBIA 



i _ n -_ _J 



TELEOSTEI PROTO-PENTADACTYLOIDEI 



GANOIDEI 



-DIPNOI 



L_ 



PROTO-GANOIDEI 



HOLOCEPHALI 

 ELASMOBRANCHII 



PROTO-GNA 



[- 



HOSTOMATA 



Cydostomata PROTO-VERTEBRATA 



PROTOCHORDATA Urortorda 



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

 The other groups are all in existence at the present day, hut 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 (i) a 



1 The openings of the hepatic diverticula through the sacks lined with thread cells 

 are described by Hancock and Embleton, Ann. and Mag. of Nat. History, Vol. xv. 

 1845, p. 82. Von Jhering has also recently described these openings (Zool. Anzeiger, 

 No. 23) and apparently attributes their discovery to himself. 



