ABERRANT FORMS OF LAMELLIBRANCHIATE GILLS. 597 
is quite pointed. The resemblance of the gill to a boat is, 
however, only very superficial, as the gill is not one solid 
mass, but is made up of a series of paired plates of a peculiar 
shape, placed one after another from the anterior to the pos- 
terior end. A little dissection under a lens will show that 
the part above the line zdp (fig. 4) and below the line of 
suspension (xcp), is continuous along the entire length of 
the gill, and that, with this part for the stem, the plates are 
given off, one after another, in pairs to the two sides (see 
fig. 5). The plates constitute the proper respiratory parts of 
the organ. They are largest in the middle, and diminish 
in size toward the two extremities. 
It is evident from this description that the gill in Nucula 
is of quite an exceptional nature. It does not, as in most 
Lamellibranchs, extend along the whole length of the side 
of the body, constituting the most conspicuous object of the 
mantle cavity, but is comparatively insignificant, being 
pushed back and freely suspended in the mantle cavity. It 
cannot, therefore, divide the latter into the suprabranchial 
and infrabranchial chambers, and is, of course, utterly de- 
void of any structure like the ciliated water-passages in the 
ordinary gill, for driving water from the lower to the upper. 
It cannot, also, as has been said, serve as an effective food- 
procuring organ. ‘The gill in Nwcula must for these reasons 
be of vastly less functional importance to the animal than it 
is in common Lamellibranchs, and, so far as I am able to 
see, serves only as the organ of respiration. It seems to me, 
however, that the division of the mantle cavity into the 
upper and lower chambers is begun in the posterior part. 
It has been seen that ventral to the membrane suspending 
the gill (m, figs. 1 and 3) there is a large space continuons 
with the general branchial cavity, and there certainly is 
a space dorsal to this membrane. These spaces seem to be 
the rudiments of the supra- and infrabranchial chambers. 
Moreover, the arrangement of the different parts at the 
posterior end, as seen in fig. 3, recalls that of the correspond- 
ing parts in many of those genera in which the mantle cavity 
is divided into two parts. It is not difficult to conceive how 
the same division might be brought about in the case of 
Nucula, by proper development of the gill and the mem- 
brane. 
Fig. 5 shows a pair of opposed plates considerably en- 
larged. The solid part (¢ dj) which I have called the stem, 
and which is continuous throughout the whole length of the 
gill, together with the suspending membrane (4 77 /) is seen 
in cross section in the middle, and from this middle portion 
VOL, XXI.—-NEW SER. RR 
