294 THE VITAL FUNCTIONS. 



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 Puhnone Marino of the 

 Italians), is derived from the supposed resem- 

 blance 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 respiratory organs, chiefly, 

 I believe, because they are not known to serve 

 any other purpose. 



The JEjitozoa, in like manner, present no ap- 

 pearance of internal respiratory organs ; so that 

 they probably receive the influence of oxygen 

 only through the medium of the juices of the 

 animals on which they subsist. PlanariiB, which 

 have a more independent existence, though en- 

 dowed with a system of circulating vessels, have 

 no internal respiratory organs ; and whatever 

 respiration they perform must be wholly cuta- 



* See the delineation of this animal in Fig. 135, vol. i. p. 276. 

 t Pages 86 and 87 of this volume. 



