126 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



gradually added on to the primitive Tunicate type, as adaptations to a 

 sedentary life. If Salpa has been evolved from a swimming ancestor 

 like Appendicularia through an uninterrupted series of free pelagic 

 stages, we can give no explanation whatever of its Ascidian type of 

 structure, while this is perfectly intelligible on the view that it is a 

 modified Ascidian. 



SECTION 2. Views on the Relationships of the Swimming Tunicates. 



We must now discuss its systematic relation to the other swimming 

 Tunicates, and the nature of its relation to the Ascidians. 



The Tunicates which are most like salpa are Pyrosoma, Anchinia, 

 Dolchinia, Doliolum, and Octacnemius. We know too little of Octacne- 

 mius to make much use of it, and if it is related to salpa at all it is not 

 a stem form, but a salpa which has been secondarily modified, so we 

 need give it no more attention at present. If Dolchinia is not a Dolio- 

 lum, it is so near to Doliolum that it throws no new light on the origin 

 of Salpa. 



We do not know the whole life-history of Anchinia, but our know- 

 ledge of it is sufficient to show the justice of the opinion which, so far as 

 I am aware, is universally accepted, that it is very closely related to 

 Doliolum, and our comparison may therefore be narrowed down to 

 Pyrosoma, Salpa, and Doliolum, including with the latter Anchinia and 

 Dolchinia. 



The student of the recent literature on the systematic relationships 

 of these three groups finds total confusion and irreconcilable contradic- 

 tion. 



I shall not attempt to discuss all the opinions on record, as three 

 selections are enough to show how little is definitely established. I shall 

 therefore confine myself to the views of Grobben (Doliolum. Arbeiten 

 Zool. Inst., Wien, IV, 2, 1882), Uljanin (7), and Herdman (14), and before 

 I discuss the statements of these authors I will state as briefly as pos- 

 sible the distinctive characteristics of each. 



Grobben holds (p. 67) that Pyrosoma, Doliolum and Salpa are very 

 closely related ; that they form a natural group, and that they have been 

 derived from the Compound Ascidians, through a Pyrosoma-like ancestor. 

 So far as they are contained in this outline, I myself accept Grobben's 

 views, except that I doubt whether the Ascidian ancestor had the dis- 

 tinctive characteristic of any modern group of Ascidians. 



