538 PHILOSOPHY OF ZOOLOGY. 



rity. The peritoneal membrane appears destined to exe- 

 cute the same functions *. 



There are three remarkable modifications of the aerating 

 organs. In the first, exemplified in the Crustacea, they 

 are gills, in the form of tufts of plumes, to which the water 

 has direct access. In the second, exhibited in part of the 

 Arachnida, the lungs are in the form of sacs or lateral 

 pouches, having the bloodvessels spread upon the walls. 

 In the third group, including Insects and some Arachnida, 

 there are, on each side of the body, small orifices, termed 

 stigmata, differing in number according to the species. 

 These are formed by a cartilaginous ring, and, in some 

 cases, furnished with one or more valves. Each orifice is 

 the extremity of a short tube, which opens internally into 

 a cavity, one on each side of the body. From these la- 

 teral cavities arise innumerable tubes, termed trachete, most 

 numerous at the termination of the stigmata, which convey 

 the air to every part of the body. To enable them to do 

 this, their coats consist of an external and internal cellular 

 membrane, with a middle layer, consisting of a cartilagi- 

 nous string, spirally twisted, resembling the spiral tubes of 

 plants. These tracheae, by their number and subdivisions, 

 convey air to every part of the body, and form, indeed, the 

 great bulk of its contents. In what manner the vitiated 

 air is expelled, has not been ascertained. 



From this view of the nutritive system of insects, it 

 appears, that the chyle is absorbed by the inner sur- 

 face of the alimentary canal ; that it exudes from its ex- 

 ternal surface into the common cavity ; that the tracheae 



• See " Observations on the use of the Dorsal Vessel," by M, Marcel 

 DE Serres ; translated in Annals of Philosophy, vol. iv. p. 346 ; vol. v. 

 p. 191, 369; and vol. vi. p. 34. 



