88 SIMPLIFICATION OF ORGANIZATION. 



equally so in each department. In the vertebra li a, we 

 pass from the man to the eel or serpent: in the moli-usca, 

 from the cuttle-fish to the barnacle or oyster : in the ar- 

 TicuLATA, from the crab or lobster to the earthworm or 

 leech : in the radiata, from the star-fish or medusa to an 

 animalcule of infusions. 



The same progression is observable in each class ; in the 

 mammalia, for example, we descend from man to the whale 

 or seal. 



A cursory general survey of the animal kingdom will shew 

 us the gradual steps by which this simplification of the or- 

 ganization is effected. 



The internal articulated skeleton, on which the figure, 

 motions, and other important properties of the vertebral ani- 

 mals, which possess it, so much depend, ends in the verte- 

 bral department *. In some fishes it is reduced to the state 

 of cartilage ; and in others. It is so soft, as hardly to afford 

 points sufficiently firm for support and motion. External 

 members for locomotion do not exist in some vertebral ani- 

 mals, as serpents and certain fishes. 



The eyelids and lachrymal apparatus 5 the external ear 

 and tympanum ; the organs of touch and taste ; the parts 

 called cerebrum and cerebellum ; do not extend beyond 

 this department, nor do they exist in all the animals belong- 

 ing to this division. The sympathetic nerve belongs only 

 to the vertebral department f. 



d'llist. Nat.U 19. The reasons on which the division is grounded, and the 

 principal anatomical characters of the four departments, may be seen in tlie 

 Re^nc jinhnal. Introduction, p. 57, et suiv. 



* Unless we consider as a skeleton the curious and complicated arrange- 

 ment of connected bony pieces in the asterias ; where, however, the princi- 

 pal parts of the bony fabric are not applied, as in the vertebral animals, to 

 the formation of receptacles for tlie nervous system. 



+ If the simple nervous structures in some animals of the lower orders 

 should be regarded as a sympathetic nerve, it will not materially affect our 

 view of the subject, so far as the simplification of the organization is con- 

 cerned. Treviranus regards the knotted abdominal cord of insects and 

 worms as the vertebral ganglia of the syn)pathe(ic nerve, united into a symme- 

 trical whole. To call it a spinal marrow he thinks incorrect. '* Its situation 



