AQUATIC RESPIRATION. 267 



lians), is derived from the supposed resemblance 

 of these movements to those of the lungs of breath- 

 ing animals. The large cavities adjacent to the 

 stomach, and which have been already pointed out 

 in Fig. 249 and 252,* have been conjectured to be 

 respiratory organs, chiefly, I believe, because they 

 are not known to serve any other purpose. 



The Entozoa, in like manner, present no appear- 

 ance 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. Planarice, which have a more 

 independent existence, though endowed with a 

 system of circulating vessels, have no internal 

 respiratory organs ; and whatever respiration they 

 perform must be wholly cutaneous. Such is also 

 the condition of several of the simpler kinds of 

 Anjielida; but in those which are more highly 

 organized, an apparatus is provided for respiration, 

 which is wholly external to the body, and appears 

 as an appendage to it ; consisting generally of tufts 

 of projecting fibres, branching like a plume of 

 feathers, and floating in the surrounding fluid. 

 The Lumbricus marinus, or lob-worm, | for example, 

 has two rows of branchial organs of this description, 

 one on each side of the body ; each row being com- 

 posed of from fourteen to sixteen tufts. In the 

 more stationary Annelida, which inhabit calcareous 

 tubes, as the Serpula and the Terebella, these 

 arborescent tufts are protected by a sheath, which 

 envelopes their roots ; and they are placed on the 



* Pages 78 and 79 of this volume. 



t Arenicola piscatorum (LdiXn.) Sec a delineation of this marine 

 worm in Fig. 135, vol. i, p. 247. 



