ORGANS OF RESPIRATION. 



527 



and is confined to the interior of one or two hollow 

 ramified tubular branchiae (143. B. /. /. C. /. /.) at- 

 tached by a fold of peritoneum to the sides of the abdomen, 

 and which communicate externally only by one or two ori- 

 fices leading into the cloaca (143. B./".) The parietes of these 

 internal branchiae, which ramify throughout the whole interior 

 cavity of the body, like the tracheae of insects, have distinct 



transverse and longitudinal muscular fibres, and support the 

 ramifications of the sanguiferous vessels containing the venous 

 blood of the system, like the internal pulmonary organs of 

 air-breathing animals, which they also resemble in their 

 rythmic movements of dilatation and contraction ; and many 

 larvae of aquatic insects inspire, like holothuria, through the 

 anus and the cloaca. 



