194 JOHNS HOPKINS UNIVERSITY MOEPHOLOGICAL MONOGRAPHS. 



stome-like ancestors of the tunicates, as these have gradually lost their 

 parasitic habit, and have established themselves on lifeless bodies ; but 

 the original lips have remained, and they are to be recognized in the 

 so-called sucking knobs of the ascidian larva, while the teeth of the 

 cyclostomes are supposed to be represented by "bristle-carrying end 

 knobs" upon the suckers. 



The "so-called larva" of the ascidian s is represented in almost every 

 feature of its organization by the adult, sexually mature, appendicularia. 

 No better example of the correspondence between an adult animal and 

 an ontogenetic stage in the history of another can be desired, and we 

 may feel confident that, whatever the phylogenetic history of appendi- 

 cularia has been, that of the ascidian larva has been the same. 



Nearly all of the students who have devoted themselves to the study 

 of the tunicates agree in regarding appendicularia as a persistent repre- 

 sentative of their primitive condition; but appendicularia is an active 

 swimming organism, and I have shown that its simple structure is so 

 well adapted to the needs of its pelagic life, that there can be no inherent 

 improbability in the view that it owes its origin to simple pelagic influ- 

 ences. 



Nothing whatever in its habit of life or in its structure lends the 

 least support to the view that it is a degenerated animal, and if we accept 

 it as evidence, we are forced to believe that, far from being the fixed 

 and degenerated descendants of parasitic vertebrates, the tunicates are 

 descended from free, active, pelagic animals of very simple structure and 

 minute size. 



Even Dohrn seems to admit that the ancestors of the tunicates were 

 swimming animals, for he tells us in support of his view of the homology 

 of the endostyle (Studien, etc., VIII, p. 62) that the ancestors of the 

 tunicates were "obviously " free swimming animals, and therefore in 

 the position to seize their food by hunting. "Offenbar waren sie frei 

 schwimmende Geschopfe und damit in der Lage, ihre Nahrung durch 

 Jagd selbst zu packen." 



If the tunicates are, as their embryology and comparative anatomy 

 indicate, the descendants of an ancestor which was obviously a free swim- 

 ming animal, it is surely simpler, in view of all the facts, to regard the 

 gill-slits as perforations which were originally retained and fixed by 

 natural selection as channels for the exit of the water which was taken 

 into the mouth with the food, than to refer them back to imaginary 

 segmental organs which have left no other trace of their existence in the 

 body of any known tunicata. 



