NO. 2032. ROTATORIA OF WASHINGTON AND VICINITY— HARRING. 393 



form, there can be no doubt about their specific identity. The first 

 explanation of the variation that suggested itself was the Ostwald 

 "viscosity-adaptation" theory; the temporal distribution appeared, 

 however, to contradict any simple temperature effect as the probable 

 cause. To test tliis a record of the temperature of the water was 

 taken weekly for a year, with notes on the occurrence of the two forms. 

 Beginnmg in January, the humpless form appeared first; the inter- 

 mediate and fully humped forms did not appear until the latter part 

 of June, when the temperature was 26.5° C. The humpless form con- 

 tinued to occur, although in reduced numbers, thi-oughout the summer. 

 The humped form was far more abundant and persisted until the 

 beginning of November, when the temperature had dropped to 6.5° C 

 This would seem to exclude any direct temperature effect as the con- 

 trolling factor. It is highly probable that the key to these and similar 

 variations has been supphed by the work of Powers on Asplanchna, 

 and that a change of food organisms is the cause. 



ROUSSELETIA, new genus. 

 ROUSSELETIA CORNICULATA, new species. 

 Plate 37, figs. 1-3. 



The body of this small species is fahly stout, gibbous dorsally, 

 with a slight constriction separating the head from the posterior por- 

 tion. A well-marked tail projects over the foot, wliich is rather long; 

 at its termination a single long dorsal seta is occasionally present, 

 but generally it is broken off. The two short, conical toes occupy 

 only the ventral half of the foot joint. 



The corona is nearly terminal and has a well-developed cu'cum- 

 apical band without any dorsal gap. From the unciliated apical plate 

 project two conspicuous, shghtly decurved papillae, resembUng those 

 of the genus Plcesoma. The buccal field is senucu-cular and evenly 

 ciUated, with the mouth near the ventral edge. 



The ganghon is large, with an eyespot at its posterior termination. 

 The elongate-pyriform retrocerebral sac is filled with highly refractive 

 granules, even m the two anterior branches of the duct. 



The mastax is of huge proportions, equaling fully half the length 

 of the body proper. The short esophagus leads to a large stomach 

 without separate intestine; the gastric glands are small and difficult 

 to fuid. In full grown individuals the stomach extends forward as a 

 bUnd pocket on each side of the mastax. The entire animal is colored 

 an almost opaque green, due to the presence of symbiotic Zoochlorellge. 

 The ovary is normal and the contractile vesicle rather small. The 

 two foot glands are club-shaped and of moderate length. 



The mastax is of the vhgate or "pumping" type and highly spe- 

 ciaUzed, the mallei being reduced to very slender, unjointed and 

 strongly curved rods, a short spur projecting downward near the 



