NERVOUS AND ALIMENTARY SYSTEMS. 379 



(a) Cerebro-pleural ganglia, lying above the mouth on 

 each side on the tendon of the anterior retractor 

 of the foot, connected to one another by a 

 commissure, connected to the two other pairs 

 of ganglia (b) and (c\ by long paired connect- 

 ives, and giving off some nerves to mantle, 

 palps, etc. 



(If) Pedal ganglia^ lying close together about the 

 middle of the foot, united by connectives to (a), 

 giving off nerves to the foot, and having beside 

 them two small ear-sacs, each with a calcareous 

 otolith, and with a nerve said to be derived 

 from the connective between (a) and (b). 

 ( ( -) Visceral ganglia (also called parieto-splanchnic or 

 osphradial), lying below the posterior adductor, 

 connected to (a) by two long connectives, and 

 giving off nerves to mantle, muscles, etc., and 

 to a patch of "smelling cells" {osphradhtui) at 

 the bases of the gills. 



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 " osphradium " at the base of the 

 gills, has perhaps water-testing qualities. There are also 

 " tactile " cells about the mantle, labial palps, etc. 



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 associated structures, is absent in 

 Lamellibranchs. The short wide gullet leads into a large 

 stomach surrounded by the paired digestive gland, 

 equivalent to that of the snail. Part of the food digested 

 by these juices in the stomach is compacted in autumn into 

 a "crystalline style" -a mass of reserve foodstuffs, and 

 similar but less solid material is found in the intestine. On 

 this supply the mussel tides over the winter. Some author- 

 ities, 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 



