132 DESCRIPTIONS OF PREPARATIONS. 



26. FRESH-WATER MUSSEL (Anodonta cygnea), 



Dissected so as to show the viscera in situ. 



THE animal is suspended by the apex of its foot and fastened on its 

 left side. The mouth is superior, the anus inferior ; the heart to the left 

 hand, and the foot to the right. The right mantle lobe, gills, and body 

 wall have been removed to show the stomach, coils of intestine, liver, 

 and reproductive glands in situ. The mouth lies between the anterior 

 adductor muscle and the base of the foot. It is fringed by the upper 

 and lower lips, which are prolonged into the corresponding labial tentacles, 

 not visible here. The oesophagus is wide, and leads into the stomach, 

 which has a small cavity on its dorsal wall, and in front of the pylorus 

 a depression, the entrance to a small diverticulum, or caecum, generally, 

 but incorrectly, said to lodge the 'crystalline style/ This organ lies in 

 the stomach, and reaches its full development at the approach of autumn. 

 The liver, or hepato-pancreas, surrounds this portion of the alimentary 

 canal. It looks in this preparation white, owing to the removal of its 

 natural dark colouring matter by the action of the alcohol. Its ducts open 

 into the stomach. A black bristle is passed along the first or descending 

 segment of the intestine which passes straight towards the foot. The 

 first coil curves round towards the dorsal or haemal aspect. The second 

 coil reverses this direction, and curves concentrically and ventrally by the 

 side of the first and beyond it anteriorly. The third coil passes over the 

 first segment of the intestine on its right side, and ascends towards the 

 dorsal aspect between the first segment and the first 'coil. It then runs 

 along the dorsal surface, perforating the ventricle of the heart, and finally 

 opens by an anus behind the posterior adductor muscle into the supra- 

 branchial chamber. A ridge or 'typhlosole' runs along the posterior 

 wall of the first or descending segment of the intestine. Another com- 

 mences by a swollen or club-shaped end on the ventral or neural wall 

 of the ascending segment, at the spot where it bends abruptly towards 

 the heart, and is continued as far as the anus, as may be seen here. The 

 ingestion of food depends solely on the currents of water set up by the 

 ciliated epithelium lining the alimentary canal from mouth to anus. The 

 generative gland, in this preparation yellow, is mixed up with the liver, 

 surrounds the coils of the intestine with the exception of a small portion 

 of the second coil, and extends between the ascending and descending 

 segments, and even behind the latter. It must be borne in mind that 

 both liver and genital gland are paired organs. The sexes are separate 

 in Anodonta^ and, like other Lamellibranchiata, it has no accessory 

 organs of generation such as occur in the Snail. 



The cut edges of the ventricle of the heart, which surround the 



