316 THE VITAL FUNCTIONS. 



muscles, the one set (m, m) being longer than the 

 other (m, m, m). The branchial arteries (v) are 

 seen ramifying over the inner surface of the pul- 

 monary cavities (u) on the right side, whence 

 the blood is conveyed by a corresponding set of 

 branchial veins to the dorsal vessel ; and other 

 vessels, which are ordinary veins, are seen at o, 

 proceeding from the abdominal cavity to join the 

 dorsal vessel. The membrane which lines the 

 pulmonary cavities is curiously plaited ; present- 

 ing the appearance of the teeth of a comb, and 

 partaking of the structure of gills ; and on this 

 account these organs are termed by Latreille 

 pneumo-hmichicB . Organs of a similar descrip- 

 tion exist in Spiders ; some species having eight ; 

 others four ; and some only two : but there is 

 one entire order of Arachnida which respire by 

 means of tracheae, and in these the circulation 

 is as imperfect as it is in insects. 



It may here be remarked that an essential dif- 

 ference exists in the structure of the respiratory 

 organs, according to the nature of the medium 

 which is to act upon them ; for in aquatic res- 

 piration the air contained in water is made to 

 act on the blood circulating in vessels which 

 ramify on the external surface of the filaments 

 of the gills ; while in atmospheric respiration 

 the air in its gaseous state is always received 

 into cavities, on the internal surface of which the 

 blood-vessels, intended to receive its influence, 



