538 BIOLOGY. 



abdomen would consist accordingly of thorax and abdo- 

 men, and it is this also, which carries on the principal 

 share of the respiratory process. It therefore consists 

 generally of ten rings, and has ten pairs of spiracula, 

 namely, twice five, for both thorax and abdomen. 



Or again, the abdomen may be regarded as an in- 

 testinal and sexual cavity, and the thorax may be left 

 with its own name. In that case there would be five 

 spiracles for the sex, five for the intestine, and perhaps 

 only two for the thorax. 



Would we proceed yet further; the head can then 

 only be viewed as pharynx, and consequently as neck. 



3271. In most Branchial animals that live in the 

 water, a perfect circulation is present, because, by reason 

 of their feeble amount of respiration, all the blood is not 

 consumed. This is also the case in the young of air- 

 breathing Insects, so long as they may have to grow. 

 But when they have ceased to do this, so strong an 

 amount of tension emerges in the circulation, owing to 

 the increased respiration of air, that blood remains but 

 in scanty quantity to be carried back by the veins, and 

 the arteries now for the most part convey the air in a 

 pure state, namely, uncombined with blood, as in the 

 higher animals. 



3272. As the air-tubes pass to all parts of the body, 

 like the arteries whose place they now supply, so does 

 the nutritive juice become everywhere oxydized and con- 

 verted into parenchyma or tissue. The nutritive juice 

 doubtless transudes at once through the intestine and 

 penetrates to all parts, as in the Plants. 



3273. Of the vascular system nothing at last remains 

 but the dorsal vessel, whose ramifications appear to dis- 

 appear entirely. According to its analogy with the 

 Crabs, Scorpions, and Spiders, it is the aorta. It appears, 

 as if in Insects the circulation dies off from the living 

 body. 



The whole Insect is an air-organ, an aero-vascular 

 stem. All the organs respire directly, such as the in- 

 testine, the motor fibres, nerves, sexual parts, and 



