with Observations on the General Arrangement of the Articulata. 271 



Arachnida, and the Decapods, the Crustacea, to be followed by the Myriapoda, 

 the Annelida, and the remaindei' of the Articulata. 



It may be urged against this mode of arrangement that it is not entirely in 

 accordance with some parts of the internal anatomy of these classes, especially 

 in the supposed inferiority of structure of the circulatory and digestive organs 

 in true Hexapods. But I have elsewhere shown* that this supposed inferiority 

 is not correct, and that a circulation in distinct vessels does really exist in 

 perfect insects, as in the Arachnida and Crustacea, although the vascular 

 structures are less easily detected in insects, on account of their smaller size 

 and greater delicacy, in consequence of the much smaller size of the animals 

 themselves ; while a distinct arrangement of circulatory vessels distributed 

 over the internal organs, as well as to the muscular structures, exists also in 

 the Myriapoda. As regards the anatomy of the digestive organs, many insects 

 have these parts more perfect than the Crustacea, or even the higher Mollusca, 

 as, for instance, the Orthoptera. The supposed superiority of structure of the 

 digestive apparatus in the Arachnida and Crustacea is chiefly in the more per- 

 fect development of some of the glandular appendages, as the liver, which, as an 

 excretory organ designed to separate from the circulatory fluids a greater quan- 

 tity of carbonaceous matterf than could be thrown off" readily by the branchiae 

 or pulmono-branchise, may be rendered necessary in these classes on account 

 of their peculiar habits. In regard to their nervous system. Insects appear to 

 be much superior to the Crustacea and to the Arachnida, although the contrary 

 has usually been supposed. I do not regard the mere accumulation of nervous 

 matter in any portion of the cord on the thoracic or ventral region of the body 

 as indicatory of a higher development of the animal, because such accumula- 

 tion is necessarily consequent on the size and number of the organs of loco- 

 motion which are collected more or less nearly together in some portion of 

 that region ; and because the ventral cord with its ganglia is the analogue 

 only of the medulla spinalis with its enlargements in the F'ertebrata. But I 

 am inclined to believe that the supra-oesophageal ganglia, which recent expe- 

 riments, elsewhere detailed;}:, have convinced me are alone endowed with the 

 functions of a true brain, and consequently are the true analogues of the 



* Phil. Trans. 1843, part 2. p. 243, &c. f Dr. Willis. 



X Phil. Trans. 1843, part 2. p. 264, &c. 



