266 THE VITAL FUNCTIONS. 



as the air is received by a system of air tubes, 

 denominated trachece^ or into puhnonary cavities, 

 composing the lungs. 



§ 2. Aquatic Respiration. 



Zoophytes appear m general to be unprovided 

 with any distinct channels for conveying aerated 

 water into the interior of their bodies, so that it may 

 act in succession on the nutritive juices, and after 

 performing this office, may be expelled, and ex- 

 changed for a fresh supply. It has accordingly 

 been conjectured, on the presumption that this 

 function is equally necessary to them as it is to all 

 other animals, that the vivifying influence of the 

 surrounding element is exerted through the medium 

 of the surface of the body. Thus it is very possible 

 that in Polypi, while the interior surface of the sac 

 digests the food, its external surface may perform 

 the office of respiration ; and no other mode of 

 accomplishing this function has been distinctly 

 traced in the AcalephcB. MedusEe, indeed, appear 

 to have a further object than mere progression in 

 the alternate expansions and contractions of the 

 floating edges of their hemispherical bodies ; for 

 these movements are performed with great regu- 

 larity under all circumstances of rest or motion ; 

 and they continue even when the animal is taken 

 out of the water and laid on the ground, as long as 

 it retains its vitality. The specific name of the 

 Medusa pulmo* (the Pulmone 3Iarino of the Ita- 



* See the delineation of this animal in Fig. 81, vol. i. p. 175. 



