162 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



hybridization to-day in the family. In this regard the Salpidae are 

 sharply contrasted with the Pyrosomidae which show such inter- 

 gradation between species as strongly to suggest hybridization. 



Frequent reference has been made in this paper to the compara- 

 tive stability of the solitary form and the greater divergence of the 

 aggregated zooids. The life cycle of a salp begins with the egg and 

 includes first the solitary form, then its buds. The fully formed 

 aggregated zooid is the final stage of the ontogeny. It is, of course, 

 in line with the conditions in other groups of animals to have the 

 later stages of the ontogeny more divergent among the several species 

 than are earlier stages, provided environmental conditions are uni- 

 form throughout the life cycle. 



ORIGIN OF THE SALPIDAE. 



In any taxonomic discussion of the Salpidae, reference should be 

 made to the origin of the family and its relationships to other groups 

 of the Tunicata. 



In their adult structure there seems no clear evidence of near 

 relationship between Pyrosoma and the true Thaliacea (Doliolum, 

 Anchinia, and Salpa), nor does there appear evidence of closer rela- 

 tionship of the Thaliacea to any of the compound Ascidians. The 

 evidence from the manner of budding will be discussed a little later. 



Among the attached Ascidians, both simple and compound, the 

 most archaic seem to be the Clavelinidae. Budding was probably 

 acquired as a means of reproduction soon after the habit of attach- 

 ment was formed, sedentary life among both plants and animals 

 apparently tending toward asexual reproduction. In all families, 

 in the Tunicata, in which budding occurs, it is by means of a prolifer- 

 ous stolon of the same general type, which is a prolongation of the 

 epicardial tube, with associated mesodermal strands, into a cylin- 

 drical outgrowth of the epidermal epithelium. Outgrowths from 

 the atrium may or may not be included in the stolon. 



The Simple Ascidians other than the Clavelinidae — that is, the 

 Ascidiidae, the Cynthiidae, and the Molgulidae — have no proliferous 

 stolon, though they show an epicardial tube. We have no evidence 

 to determine if this absence of a stolon is primitive or secondary in 

 these three families. At any rate, their adult structure is more 

 complex than that of the Clavelinidae, which seem the most archaic 

 sedentary members of the phylum now living. 



The Larvacea are in many features much more archaic still. They 

 have an elongated form with locomotor tail, a hollow dorsal nervous 

 axis, a notochord lying in part at least between the nervous and ali- 

 mentary tubes, and lateral skeletal muscles are present in the tail. 

 There is a typical chordato endostyle, and there are gill slits opening 

 into atrial pouches which may or may not be homologous to the 



