SALPA IN RELATION TO EVOLUTION OF LIFE. 199 



As these chapters from my memoir on Salpa are passing through 

 the press for the second time, I take this opportunity to refer briefly 

 to a paper which has apj^eared in the meantime by Willey (Studies 

 on the Protochordata, Q. J. Mic. Sc, Jan. 1893, pp. 317-360). 



While the author seems to agree with me in rejecting Dohrn's 

 view that the tunicates are degenerated fishes, he holds that the 

 ascidians exhibit, during their development, certain features of re- 

 semblance to other primitive chordata which are not exhibited by 

 appendicularia ; and he believes that these characteristics prove that 

 the ascidians are more closely related than appendicularia to these 

 protochordata. 



The features upon which he lays most emphasis are these : I. 

 The endostyle is at first vertical and pre-oral ; II. The organ of 

 fixation is a pre-oral lobe, and its cavity is the pre-oral or anterior 

 body cavity ; and III. The first four primary stigmata of Ciona 

 intestinalis are developed from one primitive gill-slit ; the first and 

 fourth representing the two halves of one slit separated by the pre- 

 cocious development of a tongue-bar, while the third and fourth are 

 formed by constriction from the first and fourth. The six primary 

 stigmata are actually equivalent to three gill-slits, and the innumer- 

 able branchial stigmata of the adult are formed by subdivision of 

 the primary stigmata and not by new perforations. 



I cannot believe that students of the tunicata will regard the 

 first and second of these arguments as entitled to the least considera- 

 tion. It has long been known that the endostyle of ascidian larvae 

 is at first vertical or at right angles to the long axis and it is so 

 figured and described by Seeliger, but the relative position of organs 

 is so much influenced by changes in other organs that we cannot 

 attribute a phylogenetic significance to the position of the endostyle. 

 The mouth is formed very late in ascidian larvae, and as the oral 

 region of the pharynx is rudimentary when the endostyle is formed, 

 this latter organ does not occupy its true position until the floor of 

 the pharynx is pushed downwards by the development of the oral 

 region. That this is the true explanation of its change of position 

 is shown by the fact that when the mouth continues to enlarge, as 

 it does in the nutritive zooids of doliolum, the endostyle is pushed 

 past its horizontal position until it finally becomes turned upside 

 down with its long axis again at right angles to the long axis of the 



