52 MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 
among certain of the most important systems of organs, and to bring into prominence 
some facts in the anatomy of the Mollusca which have hitherto been unknown oi 
negleeted. 
With these views I propose to treat — 1, of the nervous system ; 2, of the \asculai 
system ; 3, of certain portions of the alimentary system ; and 4, of the renal system. 
1. Nervous System.— rhe nervous system of every Mollusk consists of two great 
systems;— A. an excito-motor, or sensory and volitional system; B. a visceral or 
sympathetic system. The former consists of three pairs of primary ganglia, which 
always exist, and of a variable number of accessory local ganglia, which ma) or ma} 
not exists. 
* The first record I can find of the distinct enunciation of this very important anatomical fact, is in M. Sor- 
leyet’s essay on the Pteropoda (Observations Anatomiques, Physiologiques et Zoologiques sur les Alollusques 
Pteropodes), of which an abstract is given in the Comptes Rendus for 1843 ; he says,— “ The central nervous 
system of the Mollusca is essentially composed of the three orders of ganglia which I have just pomted out 
(orders answering exactly to those mentioned in the text), and it is in fact reduced to these ganglia m a 
certain number of animals of this type. But in others the nerves which are given off present numerous enlarge- 
ments in their course, and this tendency to a ganglionic disposition is so decided among the highest MoUusbs, 
that all the nerves emanating from the central medullary masses produce new ganglia in the parts to which 
they are distributed” (p. 667). 
Ao-aln “ From the facts which have just been stated summarily, I believe I may conclude,— 
“1. 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 con- 
trary to physiological principles, but also to anatomical facts. 
“2. That the nervous system of Mollusks corresponds, in fact, in its distribution to the same parts as those 
which constitute it in the superior animals, the whole difference consisting in the degree of development and 
disposition of the parts which is in relation with the rank that Mollusks 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 Mollusks, that it is composed of ganglia scat- 
tered 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 facL are to be 
regarded only as different degrees of development of these central portions, which is proved by their degrada- 
tion 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 symmetrical, in opposi- 
tion 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 organization 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, how- 
ever, might be pardoned for this, since M. Souleyet’s owm countrymen contrive (see Blaxchaed, Sur 
I’organization des Opisthobranchies, Annales des Sc. Nat. 1848) five years afterwards not to know anytliing 
about them. 
See also the memoir of Hancock and Embleton, before cited, in which the first complete demonstration 
of the true sympathetic system in the Gasteropoda is given ; and Alder and Hancock’s British Nudibranchiata, 
in which are contained the most beautiful descriptions and figures of the anatomy of the Mollusca extant. 
