ANIMAL PHYSIOLOGY. [LESSON 79. 



with the muscular system ; the ganglion upon the posterior adductor, 

 which is most conspicuous in the Oyster, Mussel, Pecten, and other 

 Bivalves, is the largest and most constant. 



1180. It supplies the branchiae with their nerves, and is called, 

 therefore, the branchial ganglion ; but it distributes an equal share 

 of nerves to the posterior dorsal and anterior parts of the mantle. 



1181. In the common Mussel (Mytilus edulis) the labial or 

 cephalic ganglions may be distinguished by their rose color (when 

 fresh) at the base of the labial processes (a, Fig. 378). They are 

 connected by a short transverse nervous chord (6), which passes be- 

 hind the mouth. From each of these ganglions two principal nerves 

 are given off, one passing forwards to the anterior adductor (q), the 

 other backwards along the base of the foot and the visceral mass to 

 the posterior adductor (g, g). At a short distance from the cephalic 

 ganglia this latter nervous chord (g, g) sends off a branch (c, c), 

 which terminates in a ganglion at the base of the foot (d) ; as each 

 of these branches ends in a ganglion, for the supply of nerves to the 

 foot, they coalesce, and a bilobed pedal (pes, foot) ganglion is formed. 

 The cephalic (a), pedal (d) and branchial (h) ganglions, constitute 

 the important centres of the nervous system in this and other bi- 

 valves. The pedal ganglia distribute nerves in one direction to the 

 retractors of the foot; in another to the substance of the foot 

 (/) itself. The branchial ganglions (h } h) send off nerves, which are 

 distributed principally to the breathing organs (m, m), and two 

 large nerves which diverge as they pass over the adductor 

 muscle (/, J), to proceed to the base of the tentacular proceses 

 guarding the posterior lobes of the mantle ; these continue along the 

 margin of each lobe of the mantle, until they meet and anastomose 

 (join) with similar branches, which are continued over the anterior 

 adductor muscles descending from the cephalic ganglions. 



1182. The most remarkable fact demonstrated by the prepara- 

 tion of which a copy has been given, is the surprising and persistent 

 connection of the ganglionic centres ; throughout this nervous system 

 it invariably occurs that a nerve which has its origin in one ganglion, 

 terminates in another. By this means the cephalic ganglions are 

 connected, and the circuit made perfect. 



1183. The nerves (c, c), which arise from the cephalic ganglia 

 respectively, terminate in d, c?, the pedal ganglia, whilst these, in 

 their turn, are connected by the looped nerves, f, f. 



1184. The cephalic are connected with the branchial ganglia, 

 through the medium of the dorsal chords, g, g j one branchial gan- 



