CHAPTER VI. 



THE SYSTEMATIC POSITION OF SALPA. 

 SECTION 1. The Evidence that Salpa is descended from a Fixed Form. 



I formerly believed that Salpa is the modern 'representative of an 

 ancient Tunicate stem which has been pelagic throughout its whole 

 history, and has been evolved at the surface of the ocean from an 

 ancestor something like the modern Appendicularia ; and that the group 

 has nothing in common with the fixed Ascidians except a common 

 descent from this ancient form. 



While I still regard Appendicularia as the starting-point, I now feel 

 confident that in all other respects this view is wrong, and that the facts 

 force us to believe that Salpa is a modified descendant from a fixed 

 form ; that it owes nearly all the distinctive peculiarities of its structure 

 to a sedentary life ; that its adaptation to a free life at the surface is 

 secondary and comparatively recent ; and that its only connection with 

 Appendicularia is through this fixed ancestral form. It is possible, and I 

 think probable, that this fixed form was not identical with any Ascidian 

 which now exists, and it may have lacked some peculiarities which are 

 shared in common by all modern Ascidians, but it must have been either 

 a fixed Ascidian, or else an ancestor of the fixed Ascidians, with their 

 habit of life and with essentially their structure. 



The facts which have forced me to abandon my original opinion and 

 to substitute the view which has just been outlined are these. In the 

 first place, comparative anatomy forces us to believe that the atrium of 

 Salpa is identical with the perithoracic and atrial chamber of ordinary 

 Ascidians, and the facts of embryology show beyond question that this 

 is a real homology. 



In the ordinary Ascidians this system arises in the embryo as two 

 ectodermal invaginations, one on each side of the body at some distance 

 from the middle line, which grow inwards towards the pharynx, and 

 become the perithoracic vesicles, which ultimately establish a communi- 

 cation with the pharynx through the gill-slits. The two perithoracic 

 vesicles or lateral atria approach each other on the middle line, dorsal 



