ORGANS OF RESPIRATION. 531 



small round stigmata on the sides of the segments. These 

 simple internal sacs of the earth-worm and the leech, opening 

 by minute stigmata on the surface of the skin, and receiving 

 a large portion of the venous blood of the system on their 

 highly vascular mucous lining, may be adapted both for an 

 aerial and aquatic mode of respiration, or they may receive 

 air from the water itself: and there is thus a gradual transi- 

 tion from the external and internal branchial sacs of polynoe 

 and halithea, to these internal rudimentary lungs of the 

 highest pulmonated annelides, where we already observe the 

 commencement of the spiracula, and even of the tracheae of 

 the entomoid classes. 



In the entomoid classes, the respiration is generally aerial 

 and extensive, which accords with their increased muscular 

 energy, and with the high temperature they often exhibit 

 in their living and active state. In the myriapods, the most 

 vermiform of all the entomoida, the respiratory organs are 

 adapted solely for atmospheric air, and consist of ramified 

 tracheae, which commence by stigmata opening on the sides 

 of the segments, or sometimes, as in scutigera, on the 

 middle of the back, where they lead to small air-vesicles. 

 These air-tubes in the scolopendra, open on each side of 

 the alternate segments along the whole extent of the body, 

 and they divide immediately at their origins in the stigmata, 

 into numerous large branches, which form partial anasto- 

 moses with those of the adjoining segments, without con- 

 stituting, as supposed by Meckel, the large continuous 

 lateral trunks seen in the tracheae of insects. 



Insects are generally provided, in the larva as well as in 

 the adult state, with nine or ten pairs of valvular spiracula, 

 defended externally by firm margins, sphincter apparatus, 

 and surrounding converging hairs, disposed on the sides 

 of most of the abdominal and thoracic segments, and leading 

 to tracheae which ramify through every point of the system. 

 They exhibit rithmic contractions and expansions of the 

 abdominal cavity during respiration, as in pulmonated ver- 

 tebrata; and many, especially of the neuropterous larvae, 

 possess also branchiae for aquatic respiration, which are 

 sometimes deciduous and sometimes permanent, as in the 

 caducibranchiate and perennibranchiate amphibia. The 

 branchiae appear to separate air from the water and convey 



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