124 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



maxima (figs. 64 and 65, p. 84), in which some specimens show iso- 

 lated ventral moieties of the intermediate and ventral oral retractor 

 muscles. 



TTietys vagina shows very well a feature that often helps in deter- 

 mining the true relation of muscles. Each muscle in a Salpa is 

 formed upon a blood sinus, and when a muscle is interrupted the 

 sinus is generally continued across the interval, showing the real 

 relations. In comparing embryos and adult solitary individuals, 

 or younger and older aggregated zooids, one sometimes finds in one 

 a muscle, while in a corresponding position in the other there is 

 only the muscle blood sinus. The same difference may be observed 

 between divergent adult individuals, as for instance, note the abortive 

 first sphincter of the upper lip (u. 1) in figure 114 and the blood sinus 

 in a similar position in figure 115. 



Fig. 116.— Thetys vagina, solitary form, atrial muscles viewed from the right side, 

 diameters. (drawn by hoyt s. hopkins.) 



X 3 



The atrial siphon is a broad tube and bears 11 delicate sphincter 

 muscles (fig. 116) on its upper and lower lips. These unite at each 

 angle of the aperture to form a rather weakly developed short atrial 

 retractor muscle. For so large an animal the whole musculature is 

 very weak. There are lateral interruptions of several of the atrial 

 sphincters, both above and below the retractor. 



From near the posterior end of the body there protrudes, on each side, 

 a cylindrical curved protuberance, generally from a fourth to a fifth as 

 long as the whole body (fig. 114). This contains a tube whose epi- 

 thelial walls are continuous with the mantle epithelium. These 

 tubular protuberances closely resemble the "tentacles" of Traustedtia 



