446 ANIMAL RESPIRATION 



fore said to be unsegmented. The skin is also clothed with 

 cilia, by means of which a gliding sort of locomotion is effected. 

 Breathing is in all cases effected through the skin, and in the 

 aquatic Planarians (which constitute the large majority of the 

 group) the air dissolved in water is utilized for the purpose, 

 while in the land-forms damp air is the breathing medium. We 

 have in fact exactly the same sort of transition from one kind 

 of breathing to the other as in the case of leeches. Land 

 Planarians are found all over the world, especially in the tropical 

 regions, where they attain a considerable size (as much as 18 

 inches in length) and are often brilliantly coloured. One such 

 large form (Bipalium Kewense) has been accidentally imported 

 in the earth surrounding the roots of tropical plants into Kew 

 Gardens and many other places. In reference to the compara- 

 tively small and inconspicuous European species, Gamble remarks 

 (in The Cambridge Natural History]'. "In Europe there are 

 only two or three indigenous Land Planarians, of which Rhyncho 

 desmus terrestris is the most widely distributed, and has been 

 found in moist situations for the most part wherever it has been 

 looked for. It measures about % inch in length, and is dark 

 gray above, whitish below, and bears a pair of eyes near the 

 anterior extremity." In regions which have alternate wet and 

 dry seasons the Land Planarians tide over the latter, and protect 

 themselves from being dried up, by burrowing into the ground 

 and surrounding themselves with a sort of case made of hardened 

 slime. A similar protective device has been noticed in regard 

 to Earthworms, which during a dry summer may sometimes be 

 found twined together in a sort of ball deep down in the ground. 



