THE RESPIRATORY SYSTEM. 59 



creased or diminished in accordance with the needs of the 

 ecoaomy. 



It is probable that the water-vascular system and the seg- 

 mental organs of Turbellarians and Annelids, the cloacal 

 tubes of the Gephyrea and of some Holothuridea, the ambu- 

 lacral vesicles of the Echinoderms, and the large pharyngeal 

 cavity of the Polyzoa, to a greater or less extent, subserve 

 respiration, and constitute internal respiratory organs. 



In Myriapoda and Insecta, the tracheae — tubes which 

 open on the surface of the body and contain air, and are 

 curiously similar in their distribution to the water-vessels of 

 the worms — constitute a very complete internal aerial respira- 

 tory apparatus. 



In Arachnida, tracheae may exist alone, or be accom- 

 panied by folded pidmo7iary 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 heart. 



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

 cavities is governed by the contractions of muscles of the 

 body, disposed so as to alter its vertical and longitudinal 

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

 air is regulated by valves, placed at the external openings 

 (stigmata) of the trachea?, 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, which place its 

 cavity in communication with the exterior ; and water, taken 

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

 aerates the blood which circulates in their interspaces. 



The respiratory apparatus of Amphioxvs, of all adult 

 fishes, and of the tadpoles of the higher anurous Amphibia, 

 in a certain stage of their existence, is of an essentially simi- 

 lar character. The accessory respiratory apparatus for the 

 maintenance and the regulation of the currents of water over 

 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 (Pidmogasteropoda), the 

 lining wall of the mantle cavity becomes folded and highly 

 vascular, and subserves the aeration of the venous blood, 

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

 here a modification of the integument, and might be termed 

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

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



