LUNGS OF LYMN^A. 197 



creature from sea- water, which contains but little air, in the 

 same time as from the air directly. The Lupea, on the other 

 hand, cannot breathe in the air ; perhaps for this reason, that 

 its gills completely fill the cavity, and so the ingress of air is 

 hindered by the gill-laminae which lie too close together; most 

 fishes die quickly in the air in the same way and for the same 

 reason, because their gills collapse and the respiratory surface is 

 conspicuously diminished. 



It is, however, only an hypothesis when we speak, as above, 

 of a transformation of the gill-cavity into a lung ; for in none 

 of these cases have we ourselves observed the process or 

 attempted hitherto to induce it by experiment ; nay, we do not 

 even know whether Crustaceans living on land, and breathing 

 air as, for instance, Birgus latro or Gecarcinus fill their lungs 

 with water and breathe through water, when in the water, or 

 breathe exclusively through their gills so long as they are 

 under water. From the morphologist's point of view, however, 

 this hypothesis may be regarded as satisfactory ; for, in the first 

 place, the structure of a gill-lung is precisely such as we should 

 ascribe to an organ for breathing air, and, in the second place, its 

 position in the body is such that its derivation from the gill- 

 cavity or from some portion of it is immediately apparent. 



Moreover, and finally, there is another instance that has long 

 been known of such a transformation of a branchial cavity into 

 a lung, but whose full significance has only quite lately been 

 duly estimated. The Lymnaeidae, living in fresh water, have 

 true lungs ; they go from time to time to the surface of the 

 water to take air into these lungs, and the oxygen contained in 

 it suffices for some time as supplementary to that which they 

 absorb through the skin. Now it has long been known that 

 the lung-cavity of the young LyrnnaeaB when they escape from 

 the egg is full of water, and it apparently acts as a gill-cavity so 

 long as the animals do not find their way to the surface of the 

 water to inhale air into the gill-cavity and thus transform this 

 into a lung. Generally, it is true, this period of water-breathing 

 through gills is not of very long duration, perhaps of only a few 

 hours ; but Professor Forel, of Lausanne, has made us acquainted 

 with some experiments which prove that there are actually 



