ox THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 1 79 



into prominence some facts in the anatomy of the MoUusca which 

 have hitherto been unknown or neglected. 



With these views I propose to treat — i, of the nervous system ; 2, 

 of the vascular system ; 3, of certain portions of the alimentary system ; 

 and 4, of the renal system. 



I. Nervous System. — The nervous sytem of every mollusk consists 

 of two great systems ; — A., an excito-motor, or sensory and volitional 

 system ; B., a visceral or sympathetic sj-stem. The former consists of 

 three pairs of primary ganglia, which always exist, and of a variable 

 number of accessory local ganglia, which may or may not exist.^ 



^ The first record I can find of the distinct enunciation of this very important anatomical 

 fact, is in M. Souleyet's essay on the Pteropoda (Observations Anatomiques, Physiologiques 

 et Zoologiques sur les Mollusques Pteropodes), of which an abstract is given in the Comptes 

 Rendus for 1843: he says, — "The central nervous system of the MoUusca is essentially 

 composed of the three orders of ganglia which I have just pointed out (orders answering 

 exactly to those mentioned in the text), and it is in fact reduced to these ganglia in a certain 

 number of animals of this -type. But in others the nerves which are given o£f present 

 numerous enlargements in their course, and this tendency to a ganglionic disposition is so 

 decided among the highest moUusks, that all the nerves emanating from the central 

 medullary masses produce new ganglia in the parts to which they are distributed " (p. 667). 



Again : — " From the facts which have just been stated summarily, I believe I may 

 conclude, — 



"I. That the exclusive analogy which many naturalists have wished to establish between 

 the nervous system of the Mollusca, and one of the portions of the same system in the 

 animals of higher classes, is not only contrary to physiological principles, but also to 

 anatomical facts. 



"2. That the nervous system of moUusks corresponds, in fact, in its distribution to the 

 same parts as those which constitute it in the superior animals, the whole difference con- 

 sisting in the degree of development and disposition of the parts which is in relation with 

 the rank that moUusks occupy in the series, and the plan which nature has followed in their 

 zoological type. 



" 3. That the definition very commonly given of this system in moUusks, that it is com- 

 posed of ganglia scattered in different parts of the body-, is not exact, since the parts which by 

 their fixity ought to be considered as those which essentially constitute it, are always grouped 

 round the oesophagus. The others, in fact, are to be regarded only as different degrees of 

 development of these central portions, which is proved by their degradation or disappearance 

 in proportion as we descend in animals of this series. 



" 4. That the central nervous system of Mollusca is always double, and consequently sym- 

 metrical, in opposition to what some anatomists have advanced ; that it hardly differs in 

 this respect from the nervous system of the Articulata, except by the centralization of the 

 locomotive ganglia, a centralization which may be observed in many animals of the latter type. 



" 5. Lastly, that it has been wrongly asserted as a general rule, that the ganglia of which 

 the nervous circle of the Mollusca is composed, tend to approximate the higher the organiza- 

 tion of the animal, the position of these ganglia being essentially subordinate to that of the 

 organs which they have to innervate " (p. 669). 



The very just and admirable views here set forth seem to have met with strange neglect ; 

 foreigners, however, might be pardoned for this, since M. Souleyet's own countrymen con- 

 trive (see Blanchard, Sur I'Organization des Opisthobranchies, Annales des Sc. Nat. 1848) five 

 years afterwards not to know anything about them. 



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