736 COMPARATIVE PHYSIOLOGY. 



is both morphologically and physiologically a much greater 

 amount of concentration. 



Among the Mollusca we find that in the Lamellibranchs 

 the three sets of ganglia are of nearly equal importance. 

 There is no defined central nervous system, a fact which 

 we may correlate with the sedentary habit. The motor 

 nerves to the great retractor muscles pass out from the 

 adjacent ganglia ; that is, the cerebral ganglia innervate the 

 anterior retractor, the visceral the posterior. The closing of 

 the shell is active, and is caused by the passage of impulses 

 to the muscles along the motor nerves. The opening is 

 more passive, as the elastic ligament causes the valves to 

 gape when the muscles relax. This relaxation is caused by 

 inhibitory nerves which inhibit the action of the motor 

 nerves, and the muscles in consequence return to their 

 former condition. The inhibitory nerves to both muscles 

 pass out from the cerebral ganglia, but there is no evidence 

 to justify the assumption that these have any " brain " 

 function. The motor cells of the cerebral and visceral 

 ganglia can be stimulated through many peripheral sensory 

 nerves. The heart is innervated from the visceral ganglia, 

 but some physiologists who minimise the importance of the 

 innervation maintain that the heart's activity is largely pro- 

 toplasmic, and that the nerves have chiefly or wholly an 

 inhibitory or trophic function. 



Among the Gasteropods there is not much of special 

 interest in regard to the nervous system. 



In the Cephalopoda the supra-cesophageal mass is un- 

 doubtedly a true " brain." When it is destroyed the 

 ordinary vital functions, such as respiration, circulation, 

 &c., are unaltered ; the animal continues to respond to 

 external stimuli, but the power of " volition " is gone ; if 

 left to itself, it remains in one position until death ensues. 

 From this fact we see that the centres, or presiding nerve 

 cells, for all the automatic functions are placed elsewhere 

 than in the brain, but that this originates all the " voluntary" 

 muscular movements. Of the various centres, the respiratory 

 is located in the pleural ganglia ; from it nerves pass out 

 which end in the stellate ganglia, and are both motor and 

 sensory for the mantle. This centre is not self-acting, that 

 is, not automatic, as are the corresponding centres in higher 



