396 PH YL UM MOLL USCA. 



and leaves a furrow in its track. The muscle fibres, as in 

 the snail, are mainly of the slowly contracting non-striped 

 sort, but those of the adductor and of the heart show 

 oblique cross-striping. In that part of the adductor muscle 

 of Pecten (and some other bivalves) that effects the rapid 

 closing of the valves, and hence the swimming, the muscle- 

 fibres are transversely cross-striped, and the same is true of 

 those found in the margin of the mobile mantle. There is 

 here therefore a good instance of the connection between 

 striation and rapidity of contraction and relaxation. 



Nervous system. There are three pairs of nerve- 

 centres : 



(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. 



(b) 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 cerebral ganglion. 



(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 " (psphradiuni) 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, which 



