74 



THE NERVOUS SYSTEM OF MOLLUSKS. 



burrowing organ ; to many it aids in the execution of 

 short leaps." 



The bivalves possessing a foot, therefore, present three 

 pairs of ganglia instead of two the anterior or ' labial', 



PIG. 23. Nervous System of the Common Mussel. (After Owen.) I, Labial ganglia 

 connected by a short commissure above or in front of the mouth ; b, b, branchial 

 ganglia similarly connected, and also united by very long cords (d, d) with the labial 

 ganglia ; p, bilobed pedal ganglion sending branches to the muscular foot (r), and 

 closely connected with the 'auditory saccules' (s) ; /*, h', circum-pallial plexus; 

 y, byssus, by which the animal attaches itself to external substances. 



the posterior or 'branchial', and the inferior or 'pedal'. 

 It occasionally happens that the ganglia of the posterior 

 or even of the inferior pair may become approximated and 

 fused into one. 



