LE : EMBRYOLOGY OF CION V IN l l> : [NALIS. 



of it have beeu offered, but aone is very generally accepted among 



zoologists; The group is sharply marked off from all others by the 



-sion of certain peculiar characters, such as the chorda, gill slits, 



and hypophysis. Among the higher Metazoa the Chordates seem to 



have no near relatives. 



An ingenious suggestion, which has gained considerable currency, is 

 that a chordate is homologous with an annelid whose dorsal and ventral 

 surfaces are reversed. This "annelid hypothesis" has been ably advo- 

 cated by Dohrn ('75 and '82-'91) and Eisig ('87). An extensive adverse 

 criticism of the hypothesis has been made by Brooks ('93). Is any light 

 thrown on the question by the ontogenetic history of Tunicates! The 

 evidence from that source seems to me chiefly negative. Recent studies 

 of the embryology of Annelids and Mollusks show a truly marvellous cor- 

 respondence between the developmental processes in these two groups; 

 it is even possible to refer hack particular organs in both to homologous 

 hlastomeres, and to trace their differentiation through unmistakably sim- 

 ilar processes. No doubt is left in the mind as to the close phylogenetic 

 relationship of Annelids and Mollusks. The embryology of Chordates, 

 however, follows an altogether different course, and is as unlike that of 

 Annelids as the adult forms are different. 



It is possible that we must go as far down in the animal scale as the 

 Ccelenterates to find an ancestor common to the Chordates and any other 

 group of the higher Metazoa. The embryology of Tunicates seems to 

 me to support this view. 



Brooks ('93) has shown good reason for believing that all the principal 

 groups of Metazoa arose as small, permanently pelagic forms, such as we 

 find represented to-day, in a somewhat modified form, by Appendicularia 

 in the case of Chordates. 



Amphioxus, because of its adaptation to life on the bottom, has prob- 

 ably undergone considerable modification from the ancestral type. For 

 example, the chorda has been extended forward to the extreme anterior 

 end of the body to admit of the animal's burrowing in the sand ; a 

 marked asymmetry of the body has also arisen, and its size has doubt- 

 less greatly increased, calling for a metameric arrangement of its organs. 



The ascidian tadpole, too, has probably been somewhat modified by a 

 great shortening of the free-swimming (ancestral) period of its 

 ence ; but here the changes have probably been restricted to a suppres- 

 sion of certain processes or organs, so that those which remain are more 

 certainly ancestral than those which occur even in Amphioxus. The 

 post-larval history of Ascidians clearly exhibits a process of d< generation, 

 which of course is wholly ccenogenetic. 



