210 THE VITAL FUNCTIONS. 



Respiration, in its different modes, may be distinguished, 

 according to the nature of the medium which is breathed, 

 into aquatic or atmospheric; and in the former case, it is 

 either cutaneous, or branchial, according as the respiratory 

 organs "are external or internal. Atmospheric respiration, 

 again, is either trachea!, or pulmonary, according as the 

 air is received by a system of air tubes, or tracheae, or into 

 pulmonary cavities, or lungs. 



2. Aquatic Respiration. 



ZOOPHYTES appear in general to be unprovided with any 

 distinct channels for conveying aerated water into the inte- 

 rior of the bodies, so that it may act in succession on the nu- 

 tritive juices, and after performing this office, may be ex- 

 pelled, and exchanged for a fresh supply. It has according- 

 ly 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 ex- 

 erted through the medium of the surface of the body. Thus, 

 it is veiy 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 Jlcalepha. Me- 

 dusae, indeed, appear to have a farther object than mere pro- 

 gression in the alternate expansions and contractions of the 

 floating edges of their hemispherical bodies; for these move- 

 ments are performed with great regularity under all circum- 

 stances 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 Medu- 

 sa pulmo* (the Pulmone Marino of the Italians,) is derived 

 from the supposed resemblance of these movements to those 

 of the lungs of breathing animals. The large cavities ad- 

 jacent to the stomach, and which have been already pointed 

 out in Fig. 249 and 252,t have been conjectured to be res- 



* See the delineation of this animal in Fig. 135, vol. i. p. 198. 

 f Pages 67 and 68 of this volume. 



