vol. 2, pt. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 53 



Ritteria, new subgenus. 



This subgenus includes the species retracta, picteti, amboinensis, and 

 hexagona. 



Ritteria amboinensis may be taken as the type of the genus, for it is 

 nearer the norm of the genus than is R. hexagona, and these are the 

 only two species in which the aggregated zooid is well known. 



This subgenus is characterized by having the gut ventral in the 

 solitary form and less compacted into a nucleus than in the higher 

 Salpidae, while in the aggregated zooids of the only species whose 

 aggregated forms are thoroughly known, amboinensis and hexagona, 

 the gut is more closely coiled than in any aggregated Cyclosalpa. 

 I have named the subgenus for Prof. W. E. Ritter, who described 

 the species retracta and emphasized its resemblance to the Cyclosal- 

 pas. I have had for study specimens only of the species amboinensis 

 and hexagona in this subgenus and base my conclusions as to classifi- 

 cation largely on the studies of Apstein, Ritter, and Ihle. 



RITTERIA RETRACTA (Ritter, 1906, b). 



Cyclosalpa retracta Ritter, 1906, 6. 



Salpa amboinensis Apstein, 1906, b, not 1904, b. 



Salpa retracta Ihle, 1910. 



In none of the species of this subgenus is the gut in the solitary form 

 found dorsal to the gill, but in Ritteria retracta and R. picteti it is elon- 

 gated. 1 The esophageal aperture is a widely flaring, trumpet-shaped 

 opening at the base of the gill (fig. 25, p. 54). The esophagus bends 

 forward, then backward, much as in the aggregated Cyclosalpa virgula. 

 The intestine runs backward on the left of the esophagus, its tip be- 

 ing bent a little upward and to the right to open by the anus into the 

 atrium, just inside of the atrial aperture. The caecum extends for- 

 ward from the point of juncture of esophagus and intestine. The 

 resemblance of the gut to that of the aggregated Cyclosalpa virgula 

 is striking. The chief differences are the presence of a rudimentary 

 second caecum in Cyclosalpa virgula, the greater forward bending 

 of the distal portion of the intestine in C. virgula, and the flattened 

 instead of cylindrical form of the caecum in Ritteria retracta. 



But, observe, our comparison here is between the solitary Ritteria 

 and the aggregated Cyclosalpa. The aggregated Cyclosalpas, in 

 three species, virgula, bakeri, and jloridana, show a more or less coiled 

 intestine. The solitary Ritteria shows a similar condition — that is, 

 the aggregated Cyclosalpa and the solitary Ritteria diverge from the 

 more archaic Cyclosalpa character toward that of other Salpidae. 

 The solitary Ritteria retracta is not greatly modified in the character 

 of its gut, but the aggregated zooids, if known, would probably show 

 a still more curved and compacted gut. 



i Ritter, 1906, 6, fig. 2; Ihle, 1910, fig. 16, pi. 1. 



