1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 859 



fibres of the subintestinal nerve originating in that centre. It is 

 possible that the two bundles represent afferent and efferent 

 fibres, but there is no proof of this. The facts enumerated 

 however, are sutficient evidence that Bouvier was right in his 

 identification of the subintestinal glanglion. The sections show 

 further that the commissure between the two pleural o-anj^lia; 

 signifies much moi-e than a zygoneurous connection between the 

 right pleural and the subintestinal. The bulk of the commissure 

 is made up of two stout bundles of fibres (1 & 2 in figs. 37 and 

 38), running transversely and connecting respectively the dorso- 

 lateral and the ventral cell-groups of the i-ight and left pleural 

 ganglia. The upper bundle is related to the origins of the 

 •cerebro-pleural connectives, the lower bundle to the origins of the 

 right and left pallial and columellar nerves. It is evident that 

 the shoi-tening of the anterior part of the visceral loop and the 

 approximation of the subintestinal ganglion to the pleural centres 

 is connected with a crossing over of nei^ve-tracts belonging to the 

 symmetrical pallial centres, and that this transference i^s quite 

 independent of the zygoneurous or dialyneurous connections of the 

 visceral commissure. It is suggestive that this intimate union 

 between the right and left pallial centres occurs in a group of 

 Rhipidoglossa in which there are two subequal and sub- 

 symmetrical shell-muscles, innervated from the pleural gano-Ha. 

 I have given reasons for believing that the development of the 

 left muscle is correlated with the development and increased 

 functional importance of the operculum, and that when once it is 

 established its action must produce a tendency towards the 

 secondary symmetrical disposition of the organs connected with 

 or influenced by it. This tendency is manifest in the nerve- 

 centres. The asymmetrical centres become of less, the symmetrical 

 centres of greater, functional importance. The two shell-muscles 

 acting together as a pair, must be subject to nervous co-ordination. 

 This has been effected by the development, in conformity wdth 

 the physiological needs of the organism, but through the opei'ation 

 of natural selection, of transverse connections between the pleural 

 centres, and these, be it noted, are of two kinds. There is a con- 

 nection by w^ay of the cerebro-pleural connectives, providino- as 

 one may legitimately infer, for the co-ordination of sensory 

 impulses arriving from the cephalic sense-organs. And there is a 

 connection by way of the symmetrical pallial and columellar 

 nerves providing for the co-ordination of sensory inmulses 

 arriving from the anterior borders of the mantle and of motor 

 impulses travelling outwards to the two shell-muscles. 



These considerations suggest a fruitful field of enquiry into the 

 causes which have led, firstly, to the separation of pleural from 

 pedal centres ; secondly, to the various degi-ees of approximation 

 or separation of pleural and cerebi^^l centres ; and, thirdly, to the 

 approximation of the asymmetrical to the symmetrical pallial 

 centres with the concomitant shortening or partial suppression of 

 the viscei-al commissure. I have manj'- indications that an 



