AQUATIC RESPIRATION. 295 



the simpler kinds of An?ielida ; but in those 

 which are more highly organized, an apparatus 

 is provided for respiration, which is wholly ex- 

 ternal 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 Ltim- 

 bricns 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- 

 composed of from fourteen to sixteen tufts. In 

 the more stationary Annelida, which inhabit 

 calcareous tubes, as the Serpula and the Teredo, 

 these arborescent tufts are protected by a sheath, 

 which envelopes their roots ; and they are placed 

 on the head, as being the only part which 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 appearance 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 



* Arenicola piscatorum (Lam,). See a delineation of this 

 marine worm in Fig. 135, vol. i. p. 276. 



