450 



PHYLUM MOLLUSCA 



Nervous system. — There are three pairs of nerve- 

 centres : — 



(a) Cerebro-pleiiral 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 gangha {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 " {osphradium) 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 he the cihated, 

 vascular, and sensitive labial palps, two on each side, which 

 waft food into the mouth. The ciliary currents are very 

 complex, and effect a sifting of the large and small particles. 

 The large mouth opens immediately into the gullet, for the 

 pharynx of other Molluscs, with all its associated struc- 

 tures, is absent in Lamellibranchs. The short wide 

 gullet leads into a large stomach surrounded by a paired 

 digestive gland. 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 



