228 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



one. In others, like the mites, there are no specia- 

 lised respiratory organs, but a " vague respiration ; " 

 and, lastly, others, such as the pseudo-scorpions 

 (Chelifer), have replaced the lung - books by true 

 tracheal tubes. 



A number of Mollusca have, like the common 

 snail, replaced an aquatic method of respiration by 

 one that is aerial ; this is effected by the large distri- 

 bution of blood-vessels to a part of the mantle, which 

 becomes so attached to the sides of the body as to leave 

 only a comparatively small orifice by which air can 

 enter ; the modified mantle chamber is called the 

 " lung." This arrangement is not separated by any 

 wide gulf from that which is found in the branchiate 

 Gastropoda, for some of these have the walls of the 

 mantle cavity more or less well provided with a lung ; 

 and others, like the marsh-snail (Paludina), have both 

 gill and lung. On the other hand, the water-snail 

 (Lymnseus) has no gill at all, yet constantly lives in 

 water, and uses its air receptacle, as do some fishes 

 (page 232), as a hydrostatic organ. 



In some cases, as Semper has pointed out, certain 

 Mollusca may be truly spoken of as amphibious ; 

 Ampullaria, an ally of Paludina, has been observed 

 by him to use its gills and lungs in rapid alternation ; 

 " for a certain time they inhale the air at the surface 

 of the water, forming a hollow tube by incurving the 

 margin of the mantle, so that the hollow surface is 

 enclosed against the water, and open only at the top. 

 When they have thus sucked in a sufficient quantity 

 of air, they reverse the margin of the mantle, opening 

 the tube, into which the water streams. The changes 

 are tolerably frequent, once or twice in a few minutes, 

 depending, probably, on the temperature. No phy- 

 siological explanation of these rhythmic alterations 

 can, however, be at present assigned." 



It is not only among the Mollusca that we have 



