42 PHYSIOLOGICAL SERIES. 



thoracic, the second a compound mass that represents a 

 1'usion of the rneso- and inetathoracic ganglia with the first 

 two abdominal. In the abdomen there are five separate 

 ganglia, the last of which is double and is formed by the 

 fusion of two larval ganglia. 



Viallanes, Ann. Sci. Nat., ser. 7, t. ii. 1887, p. 1 

 (Brain). 



MOLLUSCA. 



v. Ihering, Anat. des Nervensystemes .... der Mollusken, 



1877. 



Garstang, ' Science Progress,' vol. v. 1896, p. 38. 

 The central nervous system of the Mollusca may be compared 



with that of an Annelid or Arthropod in so far as it is essenti- 

 ally bilaterally symmetrical and consists of a paired prse-oral 

 centre united around the oesophagus to a ventral system below the 

 gut. But in Molluscs the ventral system, when longitudinally 

 elongated, shows no true metameric segmentation, and thus di tiers 

 fundamentally from the ventral chain system of Annelids and 

 Arthropods, although it much resembles the ventral cords of a 

 Turbellarian. In addition to the prae- and postoral centres 

 common to most Invertebrates, there are present others peculiar 

 to the Mollusca. The most important of these are certain 

 pallio-visceral centres for the innervation of the mantle, gills, 

 and part- of the viscera. They occur either as a simple gunglionic 

 loop uniting the lateral parts of the circum-oral system, or as 

 a number of isolated ganglia (pleural, branchial, abdominal) 

 united together by a fibrous commissure and due apparently to 

 the disintegration of some such generalised loop. Modification 

 in the various groups of Molluses tends either toward- ilie 

 concentration of all the centres towards the head (Gfafltropodflj 

 Cephalopoda), or in the IVlervpods toward- tin- diminution 

 and -uppression of the cerebral and pedal ganglia owing n-[ rct- 

 i\<-ly to the absence of the head with its -en.-e-orgaiis and to 

 the degeneration in many forms ol' the foot or hy-.-al apparatus. 

 Ie-ide- t hi- reduction of the anterior ganglia, then- i-. in the 

 I Vice v pods, a backward migration of the jiallio-hranehial centres 

 (visceral ganglia) and a progressive inerea.-e in their si/e, lateral 



