OF EESPIEATIOX. 223 



Axolotl ; all these amphibia, like fishes, have branchial arches 

 attached to the hyoid bone, and situated at the under part of 

 the head; in the Proteus, there are three pairs of branchiae, with 

 ramified filaments, extending in the form of vascular branched 

 organs to a considerable distance beyond the branchial 

 apertures ; the water enters by the mouth and escapes by the 

 inter-branchial spaces. Besides gills, the perennibranchiate 

 amphibia possess lungs resembling the air-sacs of fishes, and 

 which we shall describe in treating of the development of these 

 organs. 



[ .385. The second form of respiratory organs, called tra- 

 chece, is met with in myriapoda, insecta, and some arachnida. 

 The tracheae are air-tubes which divide and subdivide, and be- 

 come smaller and smaller in diameter, and penetrate the sub- 

 stance of all the organs ; sometimes they are enlarged into 

 vesicular sacs, of different forms and sizes (fig. 234). These 

 tubes convey atmospheric air to the interior of all the tissues, 

 and, as they are everywhere surrounded by the blood, diffused 

 through the body of insects, a perfect aeration of that fluid is 

 effected ; the extensive ramification of the tracheae being a 

 compensation for the imperfection of their organs of circula- 

 tion. The large quantity of air contained in the bodies of in- 

 sects impart the necessary lightness and elasticity to them, 

 and the highly oxygenated condition of their circulating fluids 

 imparts energy to the muscular system, and precision and 

 activity to their movements ; to the same cause we must like- 

 wise attribute the high temperature which their bodies so 

 often acquire. Fig. 234 exhibits the respiratory system in 

 the Nepa cinerea. The air is admitted by the spiracles, or stig- 

 mata, into two great lateral tubes, which subdivide and ramify 

 through the body ; the tracheae are lined with a soft mucous 

 membrane, and covered externally with a dense, shining, 

 serous coat ; between these is interposed an elastic fibrous 

 tunic, formed of a cartilaginous filament rolled into a spiral 

 form, like the spiral vessels in plants. This admirable struc- 

 ture, affording as it does one of those striking examples of 

 creative wisdom and design, extends through all the ramifi- 

 cations of the tracheae, giving the necessary elasticity and 

 patency to tubes destined to convey air, and to ramify like 

 blood-vessels through all parts of the head, antennse/palpi, 

 legs, tarsi, wings, muscular, nervous and digestive systems ; 



