INTRODUCTION. XXI 



system is obviously adapted to the elongated forms of the 

 vermiform orders, but, from the want of concentration which 

 such an arrangement implies, this type of structure is still 

 very inferior in its character. As the Articulata become more 

 perfect in their outward form, the number of the brains be- 

 comes diminished, while their proportionate size increases; 

 and thus in the carnivorous insects, Arachnida and Crustacea, 

 they are all united into a few great masses, which, becoming 

 the general centres of the entire system, admit of a perfection 

 in their external senses, a precision in their movements, and 

 an energy of action, of which the detached character of the 

 ganglia in the lower tribes was incapable. 



This dependence of the perfection of the animal upon the 

 concentration of the central masses of the nervous system is 

 strikingly proved by the changes perceptible in the number 

 and arrangement of the ganglia, during the progress of an in- 

 sect through the different stages of its existence. In the 

 elongated body of the worm-like caterpillar each segment 

 possesses its appropriate pair of ganglia, and the consequence 

 of such diffusion of its nervous apparatus is apparent in its 

 imperfect limbs, its rude organs of sense, its sluggish move- 

 ments, and general apathy ; but as it successively attains to 

 more mature forms of existence, passing through the dif- 

 ferent metamorphoses which it undergoes, the nervous gan- 

 glia gradually coalesce, increase in power as they diminish in 

 number, until, in the imago or perfect x state, having arrived at 

 the greatest concentration compatible with the habits of the 

 insect, it is endued with new and far more exalted attributes, 

 the organs of its senses are more elaborately formed, it pos- 

 sesses limbs which previously it would have been utterly inca- 

 pable of wielding, its movements are characterized by their 

 activity and precision, and its instincts and capabilities pro- 

 portionately enlarged and exalted. 



The Homogangliate division of the animal world is ex- 

 tremely natural. Four classes of this division furnish animal 

 substances yielding medicinal agents. 



Class 1. The Crustacea, yields 

 ASTACUS FLUVIATILIS, the Cray-fish, not much used.* 



* Concretions are formed in the stomach of this fish, just before the shell is cast. 

 They consist chiefly of carbonate of lime, and were formerly employed as antacids, 

 but they are now seldom used, and consequently this fish forms no No. of this work. 



