MUSEUM OF COMPARATIVE ZOOLOGY. 49 
The Anatomy of Cyclas (Spheerium). By L. JACOBSEN. 
The soft parts of Oyclas (Sphærium) cornea, compared with the size 
of the shell, are, relatively speaking, small. Opening the valves from 
below, we find the gills on each side, and the inner walls of the valves 
lined by the mantle (Pl. III. Fig. m.), 
The mantle, composed of very delicate membranes, envelops the soft 
parts ; it is open from the anterior to the posterior adductor muscles : 
the free edge is thick, compact, free from filaments, and dark in color; 
it is prolonged over the adductors, and follows the dorsal outline of the 
animal (Pl. IIT. Fig. 1v.). At the anterior and posterior points of the 
mantle opening, and within the edges, there is a transverse membrane 
which binds them together ; at the anterior point it is small and crescent- 
shaped (PI. III. Fig. 1v. b — A) ; at the posterior point it is larger and of 
peculiar shape, it, extends from the point where the edges of the mantle 
are united to the posterior adductor, and occupies a third of the edge 
(PL TIL Fig. 1v. d, k, l; nr. e). We find in this membrane two circular 
apertures, the superior one of which is the smaller. This membrane, 
when distended, assumes the shape of a cylinder, and terminates in two 
tabular apertures, encircled by delicate filaments. "These two apertures 
are the respiratory canals, by means of which the animal takes in and 
expels the water necessary to this function (Pl. TIT. Fig. v.) ; the superior 
aperture leads to the canal of the gills, and the inferior one to the 
cavity of the mantle; the filaments encircling the apertures are very 
sensitive, the least contact causes the tubes to contract and the valves 
to close (Pl. III. Fig. rr. b). 
In addition to the adductor muscles, we have the foot, which when 
contracted has somewhat the shape of a ploughshare (Pl. III. Fig. vi. a); 
it is capable of greater distention than ig usually the case with that of 
fresh-water mollusks, and when at its greatest distention it is twice as 
long as the shell itself; posteriorly it is triangular, anteriorly it is more 
rounded, below it is quite flat; in many respects it recalls the foot of 
Helix, as well in shape as when in action ; it is white, and the anterior 
end is encircled by a beautiful rose-tinged band (PL III. Fig. 1. b). The 
foot is attached to the abdomen by means of solid and strong filaments, 
which originate on each side of the valves near the posterior adductor 
and run to the anterior adductor; the ligaments along the abdomen 
circumscribe it, and serve, probably as in other mollusks, as a means of 
attachment for the muscles of the foot and for the membrane which 
surrounds the abdomen (Pl. IIT. Fig. 1v. b). 
