22 LECTURE II. 



to demonstrate the unity of organisation between the Articulate and 

 Vertebrate animal. But tlie position of the brain is thereby reversed, 

 and the alimentary canal still intervenes in the Invertebrate between 

 the aortic trunk and the neural canal. 



The outer and the inner skeletons do agree in certain relations : 

 neither of them are primitive parts of the organism, but are modifi- 

 cations or metamorphoses of other pre-existing systems : both serve 

 as fixed points of attachment to the muscles, aid their action as levers, 

 and determine the kind of movements by particular joints : both are 

 organs of protection and support. 



But, besides the diiFerences of tissue, mode of growth and vital 

 properties, already noticed, the exo- and the endo-skeletons differ in 

 the one being developed from the skin, the other from the internal 

 cellular and fibrous systems. The exo-skeleton defends or surrounds 

 the periphery of the animal ; the endo-skeleton the intei'nal parts. 

 The exo-skeleton is related to the muscles by its inner surface, the 

 endo-skeleton by its outer surface. The exo-skeleton is the reflex of 

 the circumambient medium and relations of the animal : the endo- 

 skeleton is the index of its motive energies and its intelligence. 



Even the neural canal itself is differently constructed in the 

 segments of the exo- and endo-skeletons selected for comparison : in 

 the Vertebrate it is an arch developed from a central column {c,fig. 6.), 

 which, in like manner, gives origin to the opposite or hasmal arch ; 

 in the Articulate the neural canal is formed by processes, apodemata, 

 or entapophyses {e^fig. 5.) sent off from the great peripheral arch. 

 Moreover, the development of these processes relates rather to that 

 of tlie muscular than of the nervous systems. We saw * how greatly 

 the ganglions vary in number and position w^ow. the abdominal nerve- 

 trunks of insects. Now, if the entapophyses of the dermo-skeleton — 

 the secondary vertebrte of Carus — were developed, like the neural 

 arches of the vertebrce of the endo-skeleton, in special relation to the 

 protection of the nervous centres, and conformable in number Avith 

 the pairs of nerves thence sent offf, we ought to find them go- 

 verned by the existence and position of the ganglions ; but it is not 

 so. In the Myriapoda, as Von Baer well objects (in.), entapophyses 

 are entirely absent ; and in winged insects they are confined to the 

 thorax or locomotive segment, although there may be two, three, or four 

 ganglions in the abdomen. And, what is further to be remarked, the 

 thoracic entapophyses are not developed over the thoracic ganglions, 

 but over the inter-communicating chords. In fact, their relation to 



* Lectures on Invertebrata, 8vo, 1843, p. 205. 



f " Numerus veitebrarnm semper cum numero nervorum spinalium intimc co- 

 ha;ret." Otto Hccr, Dc Ossiuin Coiicrciionc normali, &c., 4to. 1S3(). }>. 6. 



