SALPA IN RELATION TO EVOLUTION OF LIFE. 189 



sistent representative of their primitive condition ; but appendi- 

 cularia 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 influences. 



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., YIII, p. 62) that the 

 ancestors of the tunicates were " obviously " free swimming ani- 

 mals, and therefore in the position to seize their food by hunting. 

 "■ Oifenbar 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 

 swimming 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. 



Minute pelagic animals, with soft bodies bathed on all sides by 

 pure water, do not need special organs of excretions or respiration, 

 and it is not at all probable that the pharyngeal clefts were 

 originally respiratory ; but it is easy to understand how the chan- 

 nels through which the water flowed became converted into gill- 

 slits, in accordance with the law of change of function, as the 

 descendants of the primitive tunicates grew larger and became 

 sedentary, and thus came to need respiratory organs. 



It may be argued that the thing to be explained is not the exist- 

 ence of gill-slits, but their serial reduplication or metamerism. It 

 may be held that the metameric repetition of the gill-slits of 

 ascidians forces us to regard the ascidian pharynx as the primitive 



