164 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The Salpidae have passed a little beyond the Doliolidae in structural 

 development. Their muscle bands are less regular; the branchial 

 basket is lost by the fusion of all the stigmata on each side of the 

 raphe into a single large opening, the pharyngeo-atrial partition being 

 reduced to a mere rod ; the dorsal part of the ganglion has developed 

 into an eye of a type new to the Tunicata ; and the old type of neural 

 gland, derived from the central nerve tube, is lost, and one of a new 

 sort, developed from the pharyngeo-atrial epithelium, is substituted. 

 In its structural features Salpa is the most modified of all Tunicata. 

 Its processes of budding, however, are far simpler than those of the 

 Doliolidae, and there is in Salpa no indication that it ever had and 

 has lost a very complicated series of bud forms such as the Doliolidae 

 show. Doliolum is clearly Salpa's nearest relative, but the two must 

 have diverged before the acquirement of polymorphism in the buds 

 upon the stolon. Salpa passed on to higher structural modification, 

 retaining a comparatively simple series of buds. Doliolum, remaining 

 less modified in structure, has evolved a most complicated process of 

 budding with decided polymorphism among the buds. Structurally 

 Salpa is the most highly evolved of the Tunicata. In their life history 

 the Doliolidae are the most elaborate. 



It has generally been thought that the Doliolidae, structurally the 

 more primitive branch of this last line, arose from PyrosomaAike 

 forms, the evidence usually cited being the simple character of the 

 branchial basket in the two groups and the position in both of the 

 oral and atrial apertures at opposite ends of the body. The latter 

 seems a point of little weight, for it is a simple adaptation to environ- 

 mental conditions, and might readily be independently acquired in 

 the two families. After such change in the siphons, the condition of 

 the branchial basket would be as readily derived from that of the 

 Clavclinidae as from that of Pyrosoma. 



A comparative study of the methods of budding, however, gives 

 some indication that the Doliolidae and Salpidae probably arose from 

 somewhat Pyrosoma-like forms. In Pyrosoma, Doliolum, and Salpa 

 the stolon is more complex in structure than in the Ascidians, but 

 is more primitive in the relations of the several organs to the germ 

 layers. In the Ascidians all the internal organs of the bud are derived 

 from the epicardial tube of the stolon, which is of course endodermal. 

 Even the atrium and nervous system arise from this endodermal 

 tissue. This of course is a very secondary condition. 



The Pyrosoma cyathozooid has a more primitive stolon in two 

 regards. The lateral tubes of the stolon, which give rise to the atrial 

 chambers of the buds (ascidiozooids), arise from the atrial chamber 

 of the parent, very near its aperture. These are therefore ectodermal, 

 as is natural. The nervous systems of the buds arise from rods and 



