Miscellaatous. 243 



Considerations on tJi- Kervotis /Sysfein of the Ga-steru^wda. 

 By M. II. DE Lacaze-Duthieks, 



In some notes previousljr presented to the Academy I have shown 

 that, in several aberrant types of Gasteropoda, it was always possible, 

 by means of the law of connexions, to recognize the homologous 

 parts in the innervated organs as well as in the innervant nervous 

 centres, and this notwithstanding peculiar arrangements which seemed 

 to cause the homologies to disappear by masking them. 



Before passing to the observation of fresh forms, it wiU be useful 

 to indicate some facts which must always be taken into considera- 

 tion in researches of this kind. In fact, in analyses made in order 

 to reconstruct the nervous system of the Gasteropoda, at least as it 

 seems to me that it ought to be understood, the business is to recog- 

 nize which are the aggregations of ganglionic cells of primary im- 

 portance, so as not to ascribe too high a value to ganglia which, by 

 their volume and their number, might seem to be very important, 

 although having only an accessory and secondary part to play. We 

 may rest assured that, when an organ acquires considerable propor- 

 tions, the part of the nervous system corresponding to it, which is 

 represented in ordinary cases only by a few nerves, acquires in these 

 new conditions a development proportional to that of the organ. 

 But the primai'y centre which presides over the innervation is 

 little, if at all, modified, and it is only in the peripheral portion that 

 changes take place which are often so great as to kad to the belief 

 that they have a morphological importance which does not really 

 exist. In these cases ujion the course of the principal nerves depo- 

 sits of nerve-cells, variable in number and volume, are formed, and 

 the ganglia produced by them, and which may be called strength- 

 ening or sti2:)2)h'mentary ganglia, are accessory and superadded, and 

 have not the constancy of those which compose the central system 

 properly so called. 



In investigations upon this nervous system of the Mollusca, there- 

 fore, our preoccupation must be constant ; we must endeavour to 

 recognize the ganglia of the first raiik in order to distinguish them 

 from those which, being superadded, are secondary, Nattire, in 

 multiplying these centres of secondary rank, has, as it were, dis- 

 sociated the elements. The observer, by a sort of synthesis, must, 

 on the contrary, bring together the superadded parts to reunite them 

 with those which truly constitute the central nervous system. 



There are no more demonstrative examples in support of these 

 ideas than those Avhich may be drawn from the study of the mode 

 of innervation of the digestive tube of some Gasteropod Mollusca. 

 In this mode of innervation two things are constant and invari- 

 able : — 



1, The origin of the cerebro-sympathetic connexions upon the 

 anterior and superior surface of the cerebroid ganglia, 



2. .The presence of two ganglia, usually spheroidal but always 

 symmetrical, similar and situated in the angle formed by the 



