BACKBONELESS ANIMALS WHICH BREATHE IN AIR 445 



remarkable arrangement for keeping it moist and free from germs. 

 The various organs of such worms are situated within a spacious 

 cavity (body-cavity) containing a clear lymph-like fluid, and com- 

 municating with the exterior by means of a row of valvular pores 

 placed on the middle line on the upper surface. Through these 

 the fluid can be forced out from time to time, serving not only as 

 a lubricant, but also, it would seem, as an antiseptic, checking the 

 development of the numerous germs present in the surrounding 

 earth, some of which might otherwise grow into injurious moulds, 

 &c., upon the surface of the body. 



LAND LEECHES, like almost all their aquatic relatives, breathe 

 solely by means of the skin, the blood-supply of which penetrates 

 even into the epidermis, a most unusual arrangement, but one 

 which brings the blood very close to the surface, and promotes 

 its rapid purification. This is probably correlated with the un- 

 pleasantly active habits of these creatures, regarding which 

 Semper (in Animal Lije] speaks as follows: "We know . . . 

 that there are a tolerably large number oi true aquatic animals 

 which constantly or occasionally live on land. To these, for 

 instance, belong the true land leeches, as they are called, which 

 live in the forests of India and the Indian islands, sometimes in 

 such enormous numbers that it is quite impossible for men to exist 

 in them even for a few hours. I myself have often been driven 

 out of the woods of Luzon and Mindanao [in the Philippines], 

 which are very favourable spots for insects and land-shells, by 

 the myriads of leeches living on the trees and shrubs, from which 

 they fall like a drop of dew on any human passer-by ; and I once 

 read that a whole English battalion had to beat a retreat during 

 the Sikh rebellion because they were attacked in a wood by these 

 small blood-suckers in such numbers that passing through the 

 wood was not to be thought of. They dry up with particular 

 facility; but as the air in these forests is constantly saturated with 

 moisture, even in the driest season, they live in India in the open 

 air on trees quite as well as their nearest allies, the medicinal 

 leeches, do here in Europe in the water." This quotation also 

 illustrates very well the easy transition between breathing in water 

 and breathing in damp air which some groups of animals exhibit. 



LAND PLANARIANS are members of a comparatively lowly group 

 of Flat- Worms (Turbellaria) in which the body is not made up 

 of rings or segments as in a Leech or Earthworm, and is there- 



