ABERRANT FORMS OF LAMELLIBRANCHIATE GILLS, 601 
Nucula in having a siphon, and further shows its departure 
from the ordinary lamellibranchiate structure in having a 
highly specialised tactile organ in the siphon.’ 
The gill, although different in details from that of Nucula, 
is essentially of the same structure as the latter. It is sus- 
pended by a membrane, as in Nucula. Fig. 9 shows it dis- 
sected out by itself. The line of suspension is cep; «dp 
is the ventral median line, and corresponds to zdp in fig. 
4. As in Nucula, the gill is made up of a series of paired 
plates, placed one after another, and attached to the central 
solid stem continuous throughout the whole length of the 
gill. The plates do not, however, project downward, as we 
have seen in the case of Nucula, but here turn upward (see 
fig. 11). The plates are largest in the middle, and gradually 
become smaller toward the extremities. At the front end 
(z, fig. 9) there is a rather interesting arrangement. Fig. 
10 shows diagrammatically the relations of the various parts 
at the anterior termination of the gill. It will be seen that 
the plates of the gill gradually become smaller and finally 
die out toward the front, and the gill is continued simply as 
a flat membranous structure (z, fig. 10), which goes into 
the visceral mass (v. m., figs. 9 and 10). A cross-section of 
this part shows that at its lower portion, at least, there is a 
blood-channel, probably continuous with one of the channels 
in the stem of the gill. In some specimens this membrane- 
like portion of the branchia is longer than in others, and 
goes some distance around the visceral mass. 
Owing to the rather poor state of preservation of the 
alcoholic specimens, I have not been able to make out the 
histology of the Yoldia gill as fully as I should like, but the 
following description I believe to be correct in essential 
points :—Fig. 11 represent an opposed pair of plates, and cor- 
responds to fig. 5 of the Nuweula branchia. The suspending 
membrane (47277) consists of fibres crossing each other in 
several directions, and is covered on its two surfaces by 
columnar epithelium. The solid stem (tidy) of the gill has 
two blood-channels, an upper (”) and a lower (0). The 
latter seems to be in communication with a comparatively 
free space (q) in the middle of the suspending membrane. 
Directly below the upper blood-channel (0) there is a bundle 
of tissue, which appears to be fibrous, running the length 
of the gill (seen in cross-section at f, fig. 11). It serves no 
doubt for support. The floor of the lower blood-channel (7) 
is covered by a V-shaped bundle of longitudinal fibres (s). 
1 W. K. Brooks, ‘Proc. Amer, Ass, Adv, Sci.,’ 1874 (end of note), 
