58 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



muscle I (see fig. 30, which shows a different condition in the 

 adult). The intermediate muscle and the anterior three body 

 muscles are continuous across the ventral mid line; the others 

 are interrupted ventrally. The atrial retractor and its connected 

 sphincters exactly resemble those of Ritteria retracta, but the strong 

 basal sphincter of the latter species seems to be missing in R. amboin- 

 ensis. There is close resemblance between these two species in the 

 oral muscles also. The oral retractor lies external to all but the 

 first of the muscles which it crosses. Ihle shows it external to this 

 muscle also. It connects in front with the first sphincter of the 

 lower lip and gives some fibers into the second lower lip sphincter 

 also. Most of the fibers of this sphincter muscle pass on, outside 

 the oral retractor, to form the first sphincter of the upper lip. The 

 first sphincter of the upper lip and the second of the lower lip are 

 thus connected, while the second sphincter and the very broad third 

 sphincter of the upper lip continue below into the third and fourth 

 sphincters of the lower lip. The oral retractor does not quite con- 

 nect with body muscle I. The dorsal, horizontal band between the 

 last sphincter of the upper lip and the intermediate muscle is as in 

 Ritteria retracta, being connected with neither. 



The gut is a vertical loop, moderately compact, with the anus at 

 the left of the esophageal opening. No caeca show, but the part of 

 the intestine into which the esophagus opens is enlarged, seemingly 

 something as in the aggregated zooid of Cyclosalpa floridana (fig. 15, 

 pi. 5). The problematic organ, named by Ihle a blood-forming 

 organ, is present at the left of the intestine (q, fig. 29), in a position 

 similar to that occupied by the base of the corresponding organ in 

 the other Ritterias. This organ is elongated cylindrical in Ritteria 

 retracta, shorter and approaching globular in Ihle's specimen of R. 

 picteti, and nearly spherical in R. amboinensis. In our specimens it 

 is slightly larger than in Ihle's. 



The position of these problematic organs, near the intestine, and 

 what we know of their histology (Ritter), suggests their homology 

 with the structure at the tip of the caecum in the aggregated zooids 

 of Cyclosalpa balceri (pi. 10, fig. 25, q) and C. virgula (pi. 12, fig. 30). 

 We thus have an instance of the appearance in the solitary Ritterias 

 of a feature found among the Cyclosalpas in the aggregated zooids 

 only. The most noteworthy feature in the solitary Ritteria amboi- 

 nensis is the looped character of the intestine, which is approach- 

 ing the "nucleus" condition. 



There is in the National Museum one lot of specimens, Cat. No. 

 6457, U.S.N.M. (solitary form) Albatross station D 5530, Balicasag 

 Island, between Siquijor and Bohol Islands; August 11, 1909; sur- 

 face; surface temperature, 84° F. ; 3+ specimens. 



