50 BULLETIN OF THE 



With regard to the organs contained in the abdomen we will first 

 examine the organs of digestion. The mouth in Cyclas (Sj^hcerium), as 

 in Aiiodonta, is situated on the anterior edge of the abdomen, imme- 

 diately below the anterior adductor ; it is circular, and is surrounded by 

 four triangular, sharp, flat tentacles of equal size (PI. III. Fig. iv. c, d ; 

 Fig. VIII. b, c). The stomach is surrounded by the liver. The intestine 

 makes in the first instance several curves, it then passes through the 

 ovaries to the posterior side of the abdomen, thence somewhat towards 

 the anterior and in an upward direction to the dorsal margin, where it 

 makes its last curve and then returns in a somewhat downward direction, 

 thence through the heart and over the posterior adductor, where it ter- 

 minates in the anus (PI. III. Fig. vi. /, g, h ; Fig. iv. h, o). The course 

 of the intestine may be readily traced, as its base is frequently filled with 

 a dark greenish substance, and as the membrane of the abdomen is some- 

 what transparent. 



The liver is large, it occupies the forward and upper part of the 

 abdomen, it surrounds the stomach ; its position and its structure recall 

 those of Aiiodonta, though its outline is relatively much larger (PI. III. 

 Fig. IV. p ; Fig. VI. e). 



The organs of touch have already been alluded to ; they are the 

 feelers around the mouth, the tentacula, and the filaments which encircle 

 the tracheae. With regard to their nervous system, we distinguish dis- 

 tinctly the anterior pair of ganglions, situated behind the tentacula, 

 and two filaments of nerves which follow the sides of the abdomen 

 to the posterior ganglions, which are smaller than the anterior ones 

 (PI. III. Fig. VI. a, fi, v). The membrane covering these organs is yel- 

 lowish. 



With regard to the organs of circulation, we have only been able to 

 make out the form of the heart and its situation, which are the same as 

 in Aiiodonta ; the heart may be readily seen through the membrane of 

 the mantle covering the dorsal part of the animal. As already stated, 

 the rectum runs through the heart (PI. III. Fig. iv. w). 



We now come to the respiratory organs, the gills, which in this genus 

 present a very curious formation. 



As is well known, all bivalves have two pairs of gills, the exterior 

 ones and the interior ones, which are of about the same size and run 

 parallel to each other. Consequently it is rather remarkable to observe 

 that in this genus the interior gills overlap the exterior ones, and are, 

 apparently, double their size. This difference, however, is only apparent, 

 as the interior gills stand in the same proportion in size to the exterior 



