1 78 THE EVOLI'TIOMST AT LARGE. 



and so has acquired the habit of producing 

 lungs. All molluscan lungs are very simple : 

 they consist merely of a small sac or hollow 

 behind the head, lined with blood-vessels ; 

 and every now and then the snail opens this 

 sac, allowing the air to get in and out by 

 natural change, exactly as when we air a 

 room by opening the windows. So primitive 

 a mechanism as this could be easily acquired 

 by any soft-bodied animal like a snail. Be- 

 sides, we have many intermediate links 

 between the pond-snails and my cyclostoma 

 here. There are some species which live in 

 moist moss, or the beds of trickling streams. 

 There are others which go further from the 

 water, and spend their days in damp grass. 

 And there are yet others which have taken 

 to a wholly terrestrial existence in woods or 

 meadows and under heaps of stones. All of 

 them agree with the pond-snails in having an 

 operculum, and so differ from the ordinary 

 land and river-snails, the mouths of whose 

 shells are quite unprotected. Thus land- 



