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CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 273 
of it have been offered, but none is very generally accepted among 
zoülogists. The group is sharply marked off from all others by the 
possession 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- 
ated 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- 
respondeneo between the developmental processes in these two groups; 
it is even possible to refer back particular organs in both to homologous 
blastomeres, 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 embryolog g Chordates, 
however, follows an altogether diferent course, and is as abies that of 
Annelids as the adult forms are different. 
It is possible that we must go as fur down in the animal scale as the 
Coolenterates to find an ancestor common to the Chordates and any other 
group of the higher Metazoa. The embryology of Tunieates seems to 
me to ILE t this viow. 
Brooks (93) has shown good reason for believing that all the principal 
groups of. Metazon 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, becanse of its adaptation to life on the bottom, has prob- 
ably undergone considerable modification from the ancestral type. , For 
example, the ehorda has been extended forward to the extreme anterior 
end of tho body to admit of the animals burrowing in the sand ; 
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 
orent shortening of the free-swimming (ancestral) period of its exist- 
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 degene "ation, 
which of course is wholly canogenctic 
