328 PHYLOGENY OF THE CHORDATA. 



notochord as their sole axial skeleton, (2) a ventral mouth, 

 surrounded by suctorial structures, and (3) very numerous 

 gill-slits. Two degenerate offshoots 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 per- 

 sistent. The branchial clefts had, however, become reduced in 

 number, and were provided with gill-folds ; and a secondary 

 head (vide p. 313), 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 mandibular bar into the skeleton of the jaws, 

 we come to the Proto-gnathostomata. The nearest living repre- 

 sentatives of this group are the Elasmobranchii, which still 

 retain in the adult state the ventrally placed mouth. Owing to 

 the development of food-yolk in the Elasmobranch ovum the 

 early stages of development are to some extent abbreviated, and 

 almost all trace of a stage with a suctorial mouth has become 

 lost. 



We next come to an hypothetical group which we may call 

 the Proto-ganoidei. Bridge, in his memoir on Polyodon 1 , 

 which contains some very interesting speculations on the affini- 

 ties of the Ganoids, has called this group the Pneumatoccela, 

 from the fact that we find for the first time a full development of 

 the air-bladder, though it is possible that a rudiment of this 

 organ, in the form of a pouch opening on the dorsal side of the 

 stomachic extremity of the oesophagus, was present in the 

 earlier type. 



Existing Ganoids are descendants of the Proto-ganoidei. 

 Some of them at all events retain in larval life the suctorial 

 mouth of the Proto-vertebrata ; and the mode of formation of 

 their germinal layers, resembling as it does that in the Lamprey 



1 Phil. Trans. 1878. Part II. 



