310 THE DOLABELLINAE. 



complete circle, the tip resting in the siphon. The afferent and efferent vessels 

 of the ctenidium are indicated by af and ef respectively, and all that portion 

 of the gill lying to the right of a hne joining these two points is the free portion, 

 while the remainder on the left is attached in front and behind to the body-wall. 

 The ctenidium is of the folded type, the folded portions alternating above and 

 below the gill-axis. Between each set of folds on either side is found an oval 

 opening leading into the reverse side of the corresponding folds of the opposite 

 face of the gill. These openings are represented in the figure by the row of 

 darkly shaded areas upon the dorsal surface of the ctenidium. Similar open- 

 ings are to be found, alternating with these upon the ventral side. The length 

 of the ctenidium, measured along its outer margin, is approximately 90.0 mm., 

 22.0 mm. of this being along the attached area. 



At the anterior end of the ctenidium upon its ventral surface, is a small 

 elongated depression (Plate 6, fig. 5, o.) with rather prominent boundaries, the 

 osphradium. It lies directly in the path of the inflowing current of water 

 between the parapodia. 



The external spermatic groove, i, leads back to the genital opening, h, 

 lying below the anterior portion of the ctenidium. The position of this opening 

 does not agree with that given by Pilsbry in Tryon's Manual, 16, p. 150, in 

 which the location of the genital opening beneath the posterior part of the 

 ctenidium is stated to be one of the characteristics of the Dolabellinae. A 

 similar location in front of the middle of the gill is also found in all the speci- 

 mens of D. hasseltii that I have examined. 



The anus is a pocket-like opening situated just within the low posterior 

 wall of the siphon, slightly to the right of the median hne. Its margin is scarcely 

 elevated, save in front. 



The posterior mantle-margin is continued into the siphon, (Plate 6, fig. 1, 

 5, s), the left and anterior walls of which reach a height of 7.0 mm. The pos- 

 terior wall drops down rapidly into a scarcely perceptible fold behind the anus. 

 In this form it may be traced to the right and downward to rise into a well- 

 marked fold or ridge, which continues forward on the floor of the pallial chamber 

 to the genital opening, (Plate 6, fig. 1, g). In the median line, in front and to 

 the right of the base of the siphon, a thickening in the floor of the palUal cham- 

 ber forms a nearly circular disc, or button. The thickened margin of this 

 disc is best developed on the right side, and is elevated ca. 3.5 mm. above the 

 floor of the paUial chamber, becoming shghtly less in front, (Plate 6, fig. 1, n., 

 fig. 5, c). As pointed out by Gilchrist (1894) a similar button-like elevation in 



