MECHANISM OF RESPIRATION 139 



membranes or across moist surfaces, is common 

 to all animals and even to plants as well ; but 

 the special mechanism of respiration exhibits 

 great phyletic diversity. There are four or five 

 principal methods of breathing : cutaneous, by 

 the entire surface of the body, as with earth- 

 worms, leeches, and planarians ; branchial, by 

 specialised cutaneous processes called gills or 

 branchiae, as with many marine Annelid worms, 

 some leeches (Branchellion and Ozobranchus], 

 molluscs, and Crustacea ; tracheal, by air-tubes 

 traversing the body and surrounding the viscera, 

 as with insects and spiders ; trematic, by gill- 

 clefts or visceral clefts piercing the body-wall 

 and leading from the cavity of the pharynx or 

 anterior part of the alimentary canal to the 

 exterior ; pulmonary, by lungs or air-chambers, 

 as with pulmonate molluscs (Gastropods) and 

 air-breathing vertebrates. 



These different methods of respiration are not 

 merely adaptations to the environment, but they 

 are adaptations which keep pace with evolution 

 independently of the environment. The super- 

 ficial resemblance in shape of body, swimming 

 movements, and burrowing habits, between the 

 Annelid worm Ophelia, which breathes by simple 

 filiform, cutaneous gills, and Amphioxus, which 

 breathes by gill-clefts, has been remarked by 

 Cav. Lo Bianco and by myself; they are often 

 taken together in the same sandy bottom, in the 



