MUSEUM OF COMPARATIVE ZOOLOGY. 49 



Tlie Anatomy of Cyclas (Sphasrium). By L. Jacobsen. 



The soft parts of Cyclas {Sphcnrium) cornea, compared with the size 

 of the shell, are, relatively speaking, small. Opening the valves from 

 below, we find the gills ou each side, and the inner walls of the valves 

 lined by the mantle (PI. III. Fig. in.). 



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 (PI. III. Fig. iv.). 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. IV. h-h); 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 

 (PI. III. Fig. IV. i, Jc, I; III. c). 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 

 tubular 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 (PI. III. 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 (PI. III. Fig. II. h). 



In addition to the adductor muscles, wo have the foot, which when 

 contracted has somewhat the shape of a ploughshare (PI. III. Fig. vi. a) ; 

 it is capable of greater distention than is 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 (PI. III. Fig. i. 5). 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 

 suiTounds the abdomen (PI. III. Fig. iv. h). 



