Chap. IV.] THE NERVOUS SYSTEM OF MOLLUSKS. 73 



Mollusca already described, the food which it swallows is 

 brought to the entrance of its oesophagus by means of 



ciliary currents. It has two small anterior 



or 



labial ' 



ganglia (fig. 22, a, a,) one being situated on each side of the 

 mouth. They are connected by a commissure arching over 

 it, and also by a more slender thread beneath the mouth. 

 From this lower commissure, 

 filaments {e) are given off to the 

 stomach. The anterior ganglia 

 receive nerves (/) from the labial 

 processes which are probably for 

 the most part afferent in function 

 — at all events, these processes 

 have no distinct muscular struc- 

 ture. Two long parallel commis- 

 sures (d, d) connect the anterior 

 ganglia with a single large com- 

 pound ' branchial ' ganglion [h), 



situated posteriorly, and close to Anterior or laMal gangUa ; b, 

 -. , _ posterior or branchial ganglion 



the great adductor muscle, it 

 gives off branches to this muscle, 

 to each half of the mantle, and gangUa. 

 to the gills (c, c). 



Other more active Lamellibranchs possess a muscular 

 appendage known as the 'foot', which is in relation with an 

 additional single or double nervous ganglion (' pedal'), and 

 is used in various ways as an organ of locomotion. Speak- 

 ing of the diverse uses of the foot among bivalves. Prof. 

 Owen says :* "To some which rise to the surface of the 

 water it acts, by its expansion, as a float; to others it 

 serves by its bent form as an instrument to drag 



Fig. 22. —Nervous System of an 

 Oyster. (Todd aftei Garner.) a, a, 



(double) ; /, labial nerves c, c, 

 branchial nerves ; d, d, commis- 

 sures Ijetween labial and branchi.il 



them 



along the 



sands : to a third family it is 



Lect. on Comp. Anat. of Invert. Animals," p. 505. 



