AQUATIC RESPIRATION. 293 



medium which is breathed, into aquatic or atmos- 

 pheric ; and in the former case, it is either cuta- 

 neous, or branchial, according as the respiratory 

 organs are external or internal. Atmospheric 

 respiration, again, is either tracheal, or pulmo- 

 nary, according as the air is received by a 

 system of air tubes, or trachece, or into pulmo- 

 nary cavities, or lungs. 



§ 2. Aquatic Respiration. 



Zoophytes appear in 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 exchanged for a fresh supply. It has ac- 

 cordingly 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 res- 

 piration : and no other mode of accomplishing 

 this function has been distinctly traced in the 

 Acalepha. Medusae, indeed, appear to have a 

 farther object than mere progression in the 



