vou2.pt. 2] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 145 



pair in front (1) ; two lateral pairs, one below the anterior group of 

 body muscles (2), and another below the anterior one of the pos- 

 terior group of muscles (3) ; a pair in the posterior edges of the dorso- 

 lateral crests (4) ; a pair at the sides of the atrial siphon (5) ; a pos- 

 terior pair near the mid line, ventral to the horizontal crest below 

 the atrial siphon (6) ; and a single median posterior one (7) which 

 does not actually protrude beyond the contour of the crests but lies 

 in the lesser posterior ridge at the posterior end of the body. Each 

 of these "tentacles" contains a tube of mantle epithelium, which 

 at its tip is slightly enlarged. 



The muscles are confined to the dorsal half of the body. They are 

 but slightly developed. The intermediate muscle functions in con- 

 nection with the oral muscles, as in Thetys and Pegea, and will be 

 described in connection with these muscles. There are four, or per- 

 haps five, muscles which should be accounted body muscles. The 

 anterior three of these are united across the dorsal line into a single 

 band, the group much resembling the anterior group in Thalia demo- 

 cratica (fig. 104, p. Ill), although the latter merely touch without 

 fusing. Body muscle IV is longer and stretches farther down on the 

 side of the body, bending forward at its lower end. Body muscle IV 

 is branched, its narrower posterior branch running on each side to 

 the base of the atrial siphon where it again branches into two, the 

 upper of which gives off a minute branch which runs up to connect 

 with the delicate atrial sphincter, while its larger portion passes 

 below the atrial siphon to fuse with its fellow from the other side. 

 The more ventral of the two branches formed by the second dicho- 

 tomy of muscle IV turns downward and forward toward the visceral 

 mass, which however it does not reach. 



This arrangement of the branches of the fourth body muscle 

 reminds one of the conditions in the aggregated zooids of different 

 species of Gyclosalpa, in which a posterior branch of the last body 

 muscle passes beneath the atrial siphon, giving rise there to what 

 we have described as the "visceral muscle," which in some cases is 

 double (Gyclosalpa affinis, C. fioridana) and in other cases is single 

 (G oakeri, 0. pinnata, C. virgula). Even in G. affinis and G. fioridana 

 the visceral muscle is double only at its base, at its distal end the right 

 and left halves being fused into one. But observe that the above 

 comparison is between the solitary form in Traustedtia and the aggre- 

 gated zooids in the Cyclosalpas. In the very highly modified sub- 

 genus Traustedtia, the solitary form shows a feature which, in the 

 more archaic Cyclosalpas, appears only in the aggregated zooids. 

 Even the conservative member of the life cycle, the solitary form, 

 has in this divergent subgenus become much modified. Another 

 modification of the solitary form is seen in Traustedtia and in Pegea 

 in the reduction of the number of the body muscles. 



