72 OEGAiaZATIOX AJS'D DEVELOPMENT OF ANIXIAL3 IN GEXEEAL. 



thus acquire a protected position (rectal respiration of Aesclma, 

 Libellula). 



In other respects the branchial and pulmonary respiratory pro- 

 cesses are essentially the same. In the pulinonate snails (Lymnseus), 

 the pulmonary cavity may be filled with water, and yet continue to 

 function as a respiratory organ (in the young state and also under 

 special conditions in the adult, the animal remaining permanently 

 in deep water). With this fact before us of an air-breathing surface 

 functioning as a gill, it will not surprise us to find that gills and 

 branching folds of skin, which under normal circumstances serve for 

 breathing in water, can, provided they be protected from, shrivelling 

 up and desiccation either by their position in a damp space or by 

 their copious blood supply, function as lungs, and allow their pos- 

 sessors to live and breathe on land (Crabs, Birgus latro, labyrintho- 

 branchiate Fishes). 



A rapid renewal of the medium which carries the oxygen and 

 surrounds the respiratory surfaces is of the greatest importance 

 for the gaseous exchanges. We find, therefore, very often special 

 arrangements, by which the removal of that part of the respiratory 

 medium which has been deprived of oxygen and saturated 

 with carbonic acid and the introduction of another portion con- 

 taining oxygen and free of carbonic acid, is effected. In the 

 simplest cases this renewal can, although not very efficiently, be 

 brought about by the movements of the body, or by a continuous 

 oscillation of the respiratory surfaces themselves ; a method which is 

 especially common when the gills are placed in the region of the 

 mouth and function also as organs of food prehension, e.g., the 

 tentacles of many attached animals (Polyzoa, Bracliiopoda, tubi- 

 colous Worms, etc.) Very frequently the gills appear as appendages 

 of the organs of locomotion, e.cj., of the swimming or ambulatory 

 feet (Crustacea, Annelids), the movement of which brings about 

 a renewal of the respiratory medium around the gills. The move- 

 ments become more complicated when the gills are enclosed in special 

 chambers (Decapoda, Pisces), or when the respiratory organs are 

 placed within the body, as happens in the case of tracheae and 

 lungs, in which case also a renewal of the air is effected either by a 

 more or less regular movement of neighbouring parts, or by rhyth- 

 mical contractions and dilatations of the air-chamber, constituting 

 the so-called respiratory movements. The term respiration is now 

 not only applied to these movements so obvious to the eye in air- 

 breathing animals, but also to the osmotic processes, secondarily 



