TIIK KKSPIRATORY SYSTEM. 



creased or diminished in accoidan,,. uitl, tii. n. , ,! of the 

 economy. 



It is probable that the WEtW-VMCOlai vstem ami thcieg* 

 mental organs of Turbellarians ; ,ii.! AMI, 



tubes of the (,< /.lii/nii and of sonn- //o/,,//, he ftmbll- 



lacial vesicles of the I'M.inodrrms, and the Ian 

 cavity of the 1'olyzoa, to a greater or less extent, suWrre 

 respiration, and constitute internal ropiratorv organs. 



In Myriapoda and Insects, tin- ////,-/ ,/' _ tubes 

 open on the surface of the body and cental! | are 



curiously similar in their distribution t<. i! \cssclsof 



tli- worms constitute a very complete internal al : riul respira- 

 tory apparatus. 



In Arachnidct) tracheae may exist alone, or be accom- 

 panied by folded pulmonary sacs, or the latter may exist 

 alone, as in the Scorpion. In this case, these lungs are sup- 

 plied by blood which is returning- from the h< 



In these animals, the flow of air into and out of the air- 

 cavities is governed by the contractions of muscles of tin- 

 body, disposed so as to alter its vertical and l.i:-it udinal 

 dimensions. In the higher foims, the entrance and exit of 

 air is regulated by valves, placed at the external openings 

 (stigmata) of the tracheae, and provided with muscles, by 

 which they can be shut. 



In the Enteropneusta and the Tunicata a new form of 

 internal aquatic respiratory apparatus appears. The large 

 pharynx is perforated by lateral apertures, \\hich plao 

 cavity in communication with the exterior; ami iken 



in by the mouth, is driven through these branchial clefts and 

 ae'rates the blood which circulates in their interspaces. 



The respiratory apparatus of Amphioxvs, of all adult 

 fishes, and of the tadpoles of the higher anurou> J, 

 in a certain stage of their existence, is of ai, lly simi- 



lar character. The accessory respiratory appa: the 



maintenance and the regulation of the currents of \\ 

 the gills is furnished by the visceral arches and their mus- 

 cles; and the respiratory blood flows from the heart. 



In Mollusks which live on land (Fulmogasteropoda), the 

 lining wall of the mantle cavity becomes f< -rhly 



vascular, and subserves the ae'ration of the venous blood, 

 which flows through it on its way to the heart. The lung is 

 here a modification of the integument, ami mi^ht be termed 

 an external lung. The lungs of the air-breathing Vertebrata^ 

 on the contrary, are diverticula of the alimentary canal, po- 



