368 University of California Publications in Zoology. t VoL - 6 



about one-third its width, and its dorso-ventral diameter varies 

 from one to one-half the transverse diameter, or even, in excep- 

 tional eases, to twice this diameter. The folds of the rosette are 

 not simple, but are secondarily folded, standing in marked con- 

 trast to the lateral folds, which consist of a single series of undu- 

 lations. The appearance suggests that the outer wall of the funnel 

 is more contractile than the inner, the result being the production 

 of a multitude of small secondary folds on the inner surface of 

 the funnel. The gross appearance of this type of rosette suggests 

 a finely villous surface, such as an intestinal mucosa. The large 

 folds with long axis parallel to the long axis of the body, so 

 prominent in other species, are almost obliterated here, especially 

 in preserved specimens. 



The anterior or acetabular extremity (pi. 33, figs. 7-9 ; pi. 36, 

 figs. 22, 23), is bluntly pointed and bears a lateral "fin" of tissue, 

 to a point about half-way to its tip. The acetabulum (acet.) is 

 contractile within itself to a high degree and the structure as a 

 whole can be drawn back into the body by the strong longitudinal 

 muscles. This occurs in the most strongly contracted stage ( ac( I.. 

 pi. 34, fig. 10). The body is markedly asymmetrical, owing to the 

 formation of a deep "genital notch" in the left margin (gen. 

 notch). The vaginal opening lies at the angle of this notch on 

 the ventral (canal opening) surface, appearing as a narrow slit 

 bounded by a slightly elevated ring; the penis opens dorsally in 

 a pit at the base of a very low and inconspicuous mound, about 

 two-thirds of the distance from the apex of the notch to the 

 median line (p. op., pi. 36, fig. 23). The relation of penis to 

 vagina varies within narrow limits, but in general they may be 

 said to be at practically the same antero-posterior level. 



The lateral frills extend anteriorly to about the level of the 

 genital notch, in front of which they are continued as the un- 

 f rilled "lateral fin" above mentioned. Posteriorly they extend 

 to the tip of the posterior rosette. The funnel, whose margin is 

 formed by the posterior rosette, narrows rapidly to a small canal, 

 which turns at an angle slightly greater than 90°, and opens by a 

 small aperture on the ventral (vaginal opening) surface (can. op., 

 pi. 46, fig. 76). The margin of the aperture is raised into a ring, 

 and is crenate (can. op., pi. 34, figs. 14, 15). 



