MUSEUM OF COMPARATIVE ZOOLOGY, 51 
ones as they do in Anodonta; they appear larger on account of the 
prolongation of the membranes uniting them to the mantle and to the 
abdomen. 
It is generally understood that in Acephala the gills are formed by 
two closely united membranes connected on the convex edge; at the 
upper edge of the gills these membranes separate and are attached to 
the mantle or to the mantle and the lateral parts of the abdomen ; this 
structure gives rise to a triangular canal, which follows the superior edge 
of the gills and is the true gill-canal. This gill-canal is more developed 
in the exterior pair than in the interior pair; in the interior gills it 
seems that the internal membrane of the gills, which should form the 
canal, is only attached to the interior and posterior portion of the 
abdomen, and covers the intermediate portion, without, however, being 
connected with it; the result is that the interior gills possess a longi- 
tudinal aperture situated at the uppermost portion of the abdomen. 
The canal of both pairs of gills is united at the posterior extremity of 
the gills, and forms a short common canal. The gill-canals of several 
Acephala are constructed in this manner; more especially is this the case 
in Anodonta, 
The result of our investigations on the structure of these organs in 
Acephala in general holds good also with the genus Cyclas (Spheerium), 
especially so as concerns the exterior gills, but an examination of the 
interior gills shows a modification. 
That portion of the external and internal membrane of the gills which 
forms the gill-canal, and which unites the gills to the mantle and body 
of the animal, instead of being very narrow, as is usually the case in 
Acephala, is so largo that it overlaps the gill to the extent of double its 
size; consequently, in place of a narrow gill-canal, we find a cavern. 
This cavern is anteriorly broad, posteriorly narrow, and terminates in 
a small narrow passage, which is connected with the common gill-canal, 
and is attached to the inferior aperture of the posterior transverse mem- 
brane of the mantle (Pl. III. Fig. vir., vin., 1x.); at this point there is found 
a large crescent-shaped aperture, corresponding to the lateral aperture 
of the interior gill-canal, as is the case in Anodonta, and this is due to 
the fact that the internal membrane of the canal is not attached to the 
central portion of the abdomen (Pl. III. Fig. virt. y). 
The sac formed by these membranes extends on each side far beyond 
the exterior gills, and the interior gills are attached to it; consequently 
it does not run parallel to the exterior gills, but runs from in front and 
from below, to the rear and upwards, at which point it comes in contact 
