596 Ke MITSUKURi. 
outer and the outside of the inner palpus are raised into 
numerous parallel ridges, which, as shown in the figure, 
can be seen from the outside, and do not extend to the 
lower margin. At their posterior end there are two remark- 
able structures. One of them is a hood-like structure 
(7. b., figs. 1 and 2), which is the posterior prolongation of 
the united upper edges of the inner and outer palpi. The 
other (J. a., figs. 1 and 2), lying immediately below the first, 
is a long tentacular appendage. It is a hollow tube, open, 
however, along a line on its posterior aspect, and having its 
cavity continuous with the space between the two palpi. 
As it has been seen protruded, with the foot outside of the 
shell (Woodward’s ‘Manual of Mollusca,’ p. 426), and 
since, in alcoholic specimens, a great deal of dirt and sand 
is found along its length and between the palpi from its 
base to the mouth, it is no doubt a food-procuring organ, 
probably sending a constant stream of nutritive matters to 
the mouth by means of its cilia. It is interesting to notice 
in connection with this appendage that in Nucuwla, the gills, 
unlike those of ordinary Lamellibranchs, must be practically 
useless for obtaining food, as will be evident from the fol- 
lowing descripion of them. a gn 
The gill (g., figs. 1 and 3) is comparatively small. It is 
situated quite posteriorly, and is suspended by a membrane 
(m., figs. 1 and 3), which is attached to the body along the 
broken line zyzw. It is united to the visceral mass (v. m.) 
from z to y, and to the upper part of the foot (/, fig. 3) from 
y to z (see figs. land 3). At the last point, having come 
to the median line of the body, it joins with its fellow of the 
opposite side, and they continue in this way as far as w} 
Here they separate again, each proceeding to the posterior 
tip (p) of the gill of its own side. It should be remarked 
that, as the point w is further from the median line of the 
body than the point y (fig. 3), there is a considerable free 
space beneath the suspending membrane of the gill. 
When we turn to the gill itself, we find an altogether 
unusual structure. Fig. 4 shows it dissected out and seen 
from below and slightly from one side. In general appear- 
ance it resembles a boat which is suspended by its keel. 
zecp, fig. 4 (seen in cross section at 7), fig. 5), is the line 
of attachment and corresponds to the keel; zdyp, fig. 4 
(seen in cross-section at d, fig. 5), represents the bottom 
line of the hollow of the boat. ‘The latter is bounded by 
the two surfaces zapd and cbpd (fig. 4; seen in cross- 
section at bd and ad, fig. 5). The anterior end (2, fig. 4) 
is rather blunt, while the posterior end (9, figs. 1, 3, and 4) 
