STRUCTURE OF ANODONTA. 307 



The protrusion or extension of this locomotor organ is 

 in part due to an inflow of blood, which is prevented from 

 returning by the contraction of a ring muscle round the 

 veins. In moving, the animal literally ploughs its way along 

 the bottom of the pond or river-pool, and leaves a furrow in 

 its track. The muscle fibres are of the slowly contracting 

 non-striped sort. 



Nervous System. — There are three pairs of nerve-centres, 

 for which the following titles are in all probability most 

 accurate : — 



{a) Cerebro-pleural ganglia, lying above the m^outh on 

 each side on the tendon of the anterior retractor 

 of the foot, connected to one another by a com- 

 missure, connected to the two other pairs of 

 ganglia {b) and (c) by long paired connectives, 

 and giving off some nerves to mantle, palps, etc. 

 {h) 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 (p). 

 (c) 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 " 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, vas- 

 cular, and sensitive labial palps, two on each side. A 

 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 



