117 



ON THE CLASSIFICATION OF THE ANIMAL KINGDOM, 

 BY THE DISTRIBUTION OF THE NERVES 



BY F. M. BURTON, ESQ. 

 ( Continued from page 101. ) 



No. III. 



Wk have now come to the conclusion of the second great division of 

 the Animal Kingdom, and have seen how the various orders and tribes 

 gradually increase in organic development towards the great world of 

 Articulata, each development being indicated by a similar increase of the 

 nervous system in a manner that needs no comment; and we now proceed 

 to examine the Homogangliatc division, in which we find five great families — 

 Annelida, Myriapoda, Insecta, Arachnida, and Crustacea. 



Hitherto we have had to do with animals fitted only to exist in water, 

 or substance of a fluid nature; but in the class we are now entering upon, 

 we shall find animals capable, from their advanced state of organization, 

 to subsist on land. The principal external character of this division, says 

 Professor Jones, is that "they are all of them composed of a succession of 

 rings, formed by the skin or outward integument, which, from its hardness, con- 

 stitutes a kind of external skeleton, supporting the body and giving insertion 

 to the muscles provided for the movements of the animals." In the first 

 class, the Annelidans. these rings are very numerous, and the outer covering 

 is of a soft nature. In the Myriapoda the rings become less frequent, 

 and the body harder. In the Insects there is a greater concentration of 

 the external skeleton; and still more so in the Arachnidans and Crustaceans. 

 We shall also find, and this is an important fact never to be lost sight 

 of, that in proportion as the various tribes comprising this division become 

 more condensed in their structure, so much is their nervous system developed ; 

 the long many-ringed Annelidans requiring, in fact, a distribution of gang- 

 lia to guide their different segments, while it is as obviously necessary 

 for the higher orders that their nervous system should be concentrated. 



And first as regards the Annelidans. The blood of these creatures, 

 says Professor Jones, "is remarkable for its red colour, and circulates in 

 a double system of arteries and veins; they are moreover almost all hermaphro- 

 dite." Cuvier has separated them into three distinct orders, the Abranchia, 

 Dorsibranchia, and Tubicola. The animals belonging to the first order 

 have no external respiratory apparatus, such as the common leech and 

 earth-worms. The former, Hirudo medicinalis, is a soft slippery animal, 

 possessed of considerable muscular power, but without external limbs. It 

 moves about by means of flat discs at each extremity, which act as suckers; 

 near the centre of the anterior one is situated the mouth, which is armed 



voi . VII. R 



