HISTORY OF THE WORKS OF CUVIER. 145 



is \vantin:£^ in all ; tliat of sight in many ; a single family possesses that of licariug, 

 &c., bnt they all have a complete and double system of circulation, circumscribed 

 respiratory organs, a liv^er, &c. In a word, if, by the want of a spinal raaru)w, 

 of vertebra^, of a skeleton, a great sympatlietic, &c., they differ essentially from 

 the vertebrates, they seem, by the richness of their vital organs, by their double 

 circulation, their respiration, their liver, &;c., to come immediately after them, and 

 hence to deserve to form the second of tlie four branches of the animal kingdom. 



The third, or that of the artieulafa, difiers not less from that of tlie mollusks 

 than tliese dilfer from the vcrtebraia. The animals of this branch have a small 

 brain like the mollusks, and this small brain is also situated upon the oesophagus ; 

 but, what is wanting in the molluslcs, they have a sort of spinal marrow com- 

 posed of two cords which run along the belly and unite themselves with it from 

 space to space by knots or ganglions from which issue the nerves ; and yet this 

 spinal marrow, which distinguishes theai from the mollusks, does not associate 

 them with the vertebrates, for, inversely as regards that of the vertebrates, always 

 placed above the digestive canal, it is always placed below. By an opposite 

 inversion the heart, which is below this canal in the vertehrata, is above in the 

 articulata; and what I have just said of their spinal marrow may be said of their 

 skeleton, when they have one ; it is that this skeleton, while it divorces them 

 from the mollusks, is not a feature which unites them with the vertebrates ; for, 

 inversely to that of tlie vertebrates, which is internal and covered by the muscles, 

 it is external and covers the muscles. Again, in a word, the features which sepa- 

 rate the articulata from the mollusks are essential and profound, and such as decide 

 tlie nature of beings, and the features which seem to connect them with the 

 vertebrates do so only in appearance. 



The fourth branch presents characters not less circumscribed, not less deter- 

 minate than the others. The first of these characters is that all the parts are 

 disposed around a centre, like the radii of a circle ; the sotxjnd is the degrada- 

 tion, the successive simplification of their structure. From the first character is 

 derived the name of radiata, or animals of which all the parts are radiate or 

 stellate ; and from the second that of zoophytes, or animal plants, animals which, 

 in the simplicity of their organization, approach most nearly to plants. Thus 

 the animal kingdom has four great types or forms, and a little consideration will 

 disclose that each of these general forms of the body depends on the form itself 

 of the dominant system of the animal economy ; that is, on the nervous system. 



The vertebrate animals have a trunk on each side of which all their parts are 

 symmetrically arranged ; it is because their nervous system forms a central med- 

 ullary cone, from eacli side of which proceed, in symmetrical order, the nerves of 

 all those parts. The mollusks have a mass like body ; it is becaust; their nervous 

 system lias but a confused arrangtMnent. To the body of the articulata some 

 degree of symmetry is restored, but it had been first impressed on their nerAMUs 

 system; the body is articulated externally, for the nervous system is articulated 

 in the interior; in fine, even in the radiated amnicds, whenever the last vestiges 

 of the nervous sj'stem are distinguishable it presents that star-like form which is 

 affected Viy the whole body. 



The form of the nervous system, tlujn, determines the form of the animal ; 

 and the reason of this is simph; : it is that, on the whole, the nervous system is 

 virtually the animal, and all the other systems are present only for its service 

 and sustentation. It is in nowise surprising, therefore, that, the form of this sys- 

 tem remaining the same for each enibranchincnt of the animal world, the gen- 

 eral form of each should remain the same; nor that, this form changing from ono 

 embranchment to another, the form of each embranchment should change. 



* * * Having thus seen that the modilications of the nervous system givo 

 the first rjroups, the first divisions or embranchments, it follows from the principle 

 of subordination of characters, which is but another expression for the subordi- 

 nation of the organs themselves, that the modifications of the organs of circula- 

 10 S 



