ALIMENTARY AND VASCULAR SYSTEMS. 367 



Sense Organs. 



Unlike not a few bivalves which have hundreds of "eyes" 

 on the mantle margin, Anodonta has no trace of any. The 

 ear sac, originally derived from a skin pit, is sunk deeply 

 within the foot and is of doubtful use. The " smelling 

 patch" or " osphradhan" at the base of the gills, has 

 perhaps water testing qualities. There are also " tactile " 

 cells about the mantle, labial palps, &c. 



Alimentary System. 



The mouth lies between the anterior adductor and the 

 foot, and beside it lie the ciliated, vascular, and sensitive 

 labial palps, two on each side. It opens immediately into the 

 gullet, for the pharynx of other Molluscs, with all its associ- 

 ated structures, is absent in Lamellibranchs. The short 

 wide gullet leads into a large stomach surrounded by the 

 paired digestive gland (hepato-pancreas (?) ), whose juices 

 are partly analogous with those of the Vertebrate liver and 

 pancreas. Part of the food digested by these juices in the 

 stomach is compacted in autumn into a "crystalline style" — 

 a mass of reserve food stuffs, and similar but less solid 

 material is found in the intestine. On this supply the 

 mussel tides over the winter. Some authorities, however, 

 maintain that the style is a glandular secretion, protecting 

 the lining of the gut from injury. Similar structures are 

 found in several Gasteropods. The intestine, which has in 

 part a folded wall like that of the earthworm, coils about in 

 the foot, ascends to the pericardium, passes through the 

 ventricle of the heart, and ends above the posterior adductor 

 at the exhalent orifice. 



Vascular System. 



The heart lies in the middle line on the dorsal surface, 

 within a portion of the body cavity called the pericardium, 

 and consists of a muscular ventricle which has grown round 

 the gut and drives blood to the body, and of two trans- 

 parent auricles — one on each side of the ventricle — which 

 receive blood returning from the gills and mantle. The 

 colourless blood passes from the ventricle by an anterior 



