Zoological Society. 145 



exclusively performs in many cases, is yet in others so developed as 

 to enclose a mass of viscera, viz. in the thorax. 



y. In the Testudiua we have an example of those vertebral ele- 

 ments which are usually internal, becoming external, and including 

 not only all the viscera, hut having the whole muscular systeni at- 

 tached internally, as in the Articulata, and even the limbs arising 

 from the inside instead of the outside of the thorax. 



4. It presents no difficulty that the segments of the Articulata 

 have no superior or inferior arches like vertebrae, because both the 

 spinal cord and circulatory organs which those arches are respectively 

 designed to protect are included within the body (St. Hilaire) . 



5. To the order of development of a vertebra in the lateral pro- 

 cesses for locomotion being produced subsequently to the body, we 

 have an analogous case in that the Myriapoda are at birth and for 

 some time afterwards apodal, and subsequently acquire their nume- 

 rous legs (Newport on Myriapoda, Phil. Trans." 1841). This is also 

 the case with some other articulate animals. 



Section VI. 



The brain of the Fertelrata is a modification of a series of four 

 ganglia homologous with those of the spinal cord. 



1 . In the Amphioxus that part of the cord which must be regarded 

 as the homologue of the brain, because it gives off five pair of ce- 

 phalic nerves, is only distinguished from the other part of the cord 

 by its pointed anterior extremity, its posterior part being entirely 

 like the other ganglia; even its greatest vertical diameter is not 

 greater (De Quatrefages on Amphioxus, Annales des Scien. Nat., 

 third series, vol. iv.). 



2. We have already noticed that the two large cephalic gangha of 

 the Centipede are the result of the coalescence of a series of four 

 ganglia, as they appear in the foetal condition, each of these nervous 

 centres supplying nerves to the senses. Closely corresponding with 

 this arrangement is that displayed by many of the fish, as e. g. the 

 Eel, where the brain is only a series of four closely arranged ganglia. 

 And this same original scheme seems to me traceable throughout all 

 the Vertebrata to man himself. There are, however, as the great 

 centraHzation and individuality of the organ would lead us to expect, 

 many variations and modifications, which tend at first sight to con- 

 ceal its real nature, as e. g. the removal of the olfactory ganglia to a 

 great distance from the other elements of the brain, with which they 

 onlv maintain their connexion by means of fihform crura, as in the 

 Whiting and many fish ; the amphfication of the segments of the 

 encephalon by the addition of supplementary ganglia, as the hypo- 

 aria, hypophysis, &c. as they occur in many fish, and some of which 

 are retained in the higher orders, or the cerebrum in the cartilagi- 

 nous fishes, and in all animals upwards to man, and which compara- 

 tive anatomy teaches us is only to be considered as a special appen- 

 dage to or "development of the prosencephahc ganglia ; or the ex- 

 treme development of one pair of ganglia so as to obscure the others, 



Ann. S)' Mag. N. Hist. Ser. 2. Vol. vii. 10 



