NUTRITIVE APPARATUS OP ARTICULATA. 81 



ment being, as it were, the heart for its own division. The 

 respiratory apparatus, too, is arranged with the most perfect 

 symmetry. In the lower tribes, and in the Crustacea, it 

 is adapted to act in water ; and consists of gills or branchial 

 appendages, of various forms, which are prolonged from the 

 exterior of the body. But in Insects and Spiders, which con- 

 stitute the great majority of the class, the respiration is aerial ; 

 and is performed by an apparatus consisting of a set of chambers 

 or tubes, which are dispersed or extended through the whole 

 body. By this means, the air, the blood, and the tissue to be 

 nourished, are all brought into contact at the same points ; and 

 a much less vigorous circulation is required, therefore, than 

 would otherwise be needed. The whole apparatus of Nutrition 

 is comprised within a comparatively small part of the body, 

 in the higher classes at le<ist ; and the bulk of the organs 

 composing it is never to be at all compared with that, which we 

 ordinarily find in the Mollusca. Thus the Liver, which in the 

 Oyster forms a large part of the whole substance, is often 

 scarcely discoverable in the Insect ; and where (as in the Crus- 

 tacea) its bulk is considerable, it is because the respiration, being 

 aquatic, is less active than usual, and is consequently not suffi- 

 cient to draw off the superfluous carbon from the blood (ANIM. 

 PHYSIOL. 365). The blood is usually white, as in other Inver- 

 tebrated classes; and where it is otherwise (as in some of the 

 Annelida), it is in the liquor sanguinis, and not in the corpuscles, 

 that the colour exists, these last being analogous to the colour- 

 less, and not to the red corpuscles of the blood of Vertebrata. 

 The temperature of Articulata usually varies with that of the 

 air or water they inhabit ; but in the class of Insects we find 

 many, which have the power of generating a large amount of in- 

 dependent heat ; and this is strictly proportional to the quantity 

 of oxygen converted by them into carbonic acid, by the respira- 

 tory process. All the actions of the Articulata are performed 

 with great enejrgy ; and at the time of the most rapid increase of 

 the body, the demand for food is so great, that a short suspension 

 of the supply proves fatal. Many of them, however, are capable 

 of being submitted to the influence of very high and of very low 

 temperatures, with little permanent injury^. 



