vol. 2, pt. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 165 



tubes of cells which are formed by proliferation from cells of the 

 ectodermal sheath of the stolon. These nervous systems are there- 

 fore ectodermal. 



In Salpa the atrial tubes of the stolon, which form the atrial 

 chambers of the buds, arise from the atrium of the parent, thus 

 resembling Pyrosoma. In Doliolum the conditions need further study. 

 Atrial tubes are present, and from them arise the atrial chambers 

 of the buds, but it is not known from what source in the parent, or 

 in the stolon, these atrial tubes arise. In Salpa the nervous systems 

 of the buds are derived from a neural rod and tube in the stolon, 

 and this arises by proliferation from the ectodermal sheath, at the 

 base of the stolon. The conditions are therefore as in Pyrosoma. In 

 Doliolum the nervous rudiments of the buds are at first double, one 

 coming from each atrial tube; later they unite. Doliolum is there- 

 fore aberrant in having the nervous systems of the buds arise from 

 the atrial tubes while in Pyrosoma and Salpa they arise from the 

 ectoderm at the base of the stolon. The distinction is probably not 

 fundamental, as at first it appears, for in Pyrosoma the atrial tubes 

 of the stolon arise from the ectoderm of the atrium, near where it 

 opens out to the surface ectoderm. 



Pyrosoma, Doliolum, and Salpa have well-developed mesodermal 

 strands in the stolon which give rise to the gonads and the muscles 

 (Pyrosoma?) of the buds. 



In complexity of structure of the stolon, and in the relations of 

 the organs in the buds to the germ layers, Pyrosoma, Doliolum, and 

 Salpa agree, and are in sharp contrast to the Ascidians. The former 

 three agree also in having the buds on the stolon, at least when they 

 first appear, arranged in a linear series, all with their ventral surfaces 

 toward the distal end of the stolon. These resemblances between 

 these three forms in their manner of budding seem fundamental and 

 not due to secondary convergence. We must therefore believe that 

 the Doliolum-Salpa line of descent arose from forms which had 

 already acquired the Pyrosoma type of stolon. 



Pyrosoma shows two types of colony formation — one by means of 

 migrating buds, the other by means of permanently attached buds. 

 Doliolum shows the migrating type of buds. The origin of this 

 migration in a definitively Doliolum-like form is very difficult to 

 imagine. It seems more probable therefore that Doliolum has come 

 from a colonial animal like Pyrosoma, which had already, in its 

 colony formation, developed migrating buds. It is worth noting 

 that in both the Pyrosomata ambulata and in Doliolum the buds, 

 when formed, detach from the stolon and migrate to the dorsal side 

 of their parent, there to take a position definitely oriented with 

 reference to the parent, though their orientation is different in the 

 two groups. (See Metcalf and Hopkins (1918.), p. 017). 



