BORN THE AMERICAN MONTHLY [ April, 
this latter condition present even where there is no proper differentiated organ. 
Spengel further regarded this organ as a special sense organ, used to determine 
the quality of the water furnished the gill; and further, he consider it an or- 
gan of smell as implied by the term, Dze geruchsorgan, which he applies to 
te His view of it is the one taught by Prof. E. Ray Lankester in his article 
on the mollusks, in the ninth Pao of the Encyclopedia Britannica, and he 
further applies to it the name Osphradium. Just why the organ is believed 
to serve for smell rather than taste is not made very apparent. The latter 
would seem to be a better name for its function since it is bathed in liquid. 
In looking through the descriptive accounts of the genus Crepidula, with 
reference more particularly to the anatomy of the gill, it was noticed that no 
reference was made, and no figure drawn, which represented anything which 
.could be considered at all homologous with the osphradium of forms like 
Buccinum, etc. In the earlier writings of Cuvier, Quoi, Gaimard, and others 
who have given fine figures, as well as in various later writers, as Bronn and 
others, no reference can be found to a structure comparable with the ‘ acces- 
sory gill’ of other Ctenobranchs. There is, however, in this creature, or 
certainly i inC. fornicata, a structure which is plainly the osphradium, though 
very unlike the typical form as described above. 
The te Oh) of the gill in Crepzdula fornicata was described in a former 
paper. * and may be summarized as follows :—The body is distorted from the 
form common among the Ctenobranchs, and is very considerably flattened ; 
the gill cavity is very low and broad, the mantle forming the broad roof 68 
the (chambes. The cavity is triangular in outline, with the apex pointing 
posteriorly, and with a base coed outward obtusely. Along the left side 
of the triangular roof of the chamber, from a point near the middle of the 
base to near the apex, there runs a low ridge—the branchial ridge. Thisridge 
lies so far over to the left as to leave only a very small space between it and 
body-wall, where the mantle fold arises. The gills are blade shaped, long, 
narrow filaments, which originate at one . end upon the brarichial ridge, and 
run out across the mantle cavity, but free from the mantle and from each 
other, to terminate upon the right side. The osphradium, if present, must 
be sought upon the left of the ill ridge. 
In fas situation, ined rev Gals the presence of a structure not hitherto 
described, but which is, no doubt, the osphradium or ‘ olfactory organ’ of 
Crepidula fornicata. To the naked eye it is a row of small. knobs, dis- 
tinctly perceptible when the attention is called to them, and readily seen under 
a low magnification. Itsstands upon the left side of the gill ridge in a sort of 
groove made there by the ridge. It is parallel with the branchial ridge, but 
much shorter than it, being confined entirely to the anterior part of the mantle 
cavity, chiefly to the basal portion of the triangular roof. 
This organ, the osphradium, is not, at first sight, much like the osphradium 
typical of “the Ctenobranchs. It bears no resemblance, w hatever, to a gill, 
and would not be likely to receive the name of rudimentary gill from its ap- 
pearance solely. It consists of a central axis, which is a low ridge not form- 
ing any conspicuous portion of the organ until study is made by sections. 
Upon the central axis is carried a series of papille, each one independent of 
the rest, and arising from the central axis. The papillee represent the leaves 
of the ordinary form of osphradium, while the central axis is the same struc- 
ture in each. The papille are of variable number, but about 1802-20, and 
are situated a trifle farther apart than-the gill blades; have no direct relation 
in their arrangement to that of the gill blades. Each papilla consists of two 
portions—a narrow, short stalk, and a longer, much enlarged oval tip, the 
* Studies Bio. Lab. Johns Hopk. Univ., vol. iii. 
