AQUATIC RESPIRATION. 211 



piratory organs, chiefly, I believe, because they are not 

 known to serve any other purpose. 



The Enlozoa, m like manner, present no appearance of 

 internal respiratory organs; so that they probal)ly receive 

 the influence of oxygen only through the medium of tlie 

 juices of the animals on which they subsist. Planar ix, 

 which have a more independent existence, though endowed 

 with a system of circulating vessels, have no internal respira- 

 tory organs; and whatever respiration they perform must be 

 wholly cutaneous. Such is also the condition of several of 

 the simpler kinds of Annelida; 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 ap- 

 pendage to it, consisting generally of tufts of projecting 

 fibres, branching like a plume of feathers, and floating in the 

 surrounding fluid. The Lumbricns marinus^ or lob-worm,* 

 for example, has two rows of branchial organs of this de- 

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

 posed of from fourteen to sixteen tufts. In the more sta- 

 tionary Annelida, which inhabit calcareous tubes, as the 

 Serpula and the Teredo^ these arborescent tufts are protected 

 by a sheath which envelops their roots; and they are placed 

 on the head, as being the only part w'hich comes in contact 

 with the water. 



Most of the smaller Crustacea have branchiae in the form 

 of feathery tufts, attached to the paddles near the tail, and 

 kept in incessant vibratory motion, which gives an appear- 

 ance of great liveliness to the animal, and is more especially 

 striking in the microscopic species. The variety of shapes 

 which these organs assume in different tribes is too great to 

 allow of any specific description of them in this place: but 

 amidst these varieties, it is sufficiently apparent that their 

 construction has been, in all cases, designed to obtain a con- 

 siderable extent of surface over which the minute subdivi- 



* Arenlcola piscatomm (Lam.) See a deHncation of this marine worm in 

 Fig. 135, vol. i. p. 198. 



