186 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



to it, nor would they in many cases be in a position to use 

 their organs of locomotion as such. The observed facts, how- 

 ever, do not correspond to this view, which was formerly 

 somewhat hastily adopted. We know, on the contrary, that 

 there are a tolerably large number of true aquatic animals 

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

 instance, belong the true Land-leeches, as they are called (fig. 

 53,/), 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 Min- 

 danao, 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. Even more interesting are the land Planarians. 84 They 

 breathe, like the leech, through the skin (fig. 53, d, e), and dry 

 up even more readily ; they move by means of fine, micro- 

 scopically small hairs, the cilia or flagella, which are attached 

 to the skin, and which by their peculiar motions can carry the 

 animal forward when it is surrounded by a sufficient quantity 

 of trickling water or of mucilage. On a perfectly dry surface, 

 therefore, they cannot creep about for any length of time ; the 

 rapidly drying skin would soon yield up all the moisture which 

 the cilia on the under side require for their motions. Hence 

 these creatures are always found only in very damp spots; on 

 rocks, however, as well as on trees, or even on the walls of 

 houses. A few small land Planarians, two or three species, 

 occur even in Europe, where they have already been found in 

 Denmark, Germany, Spain, and France. Planaria terrestris, 

 which was discovered at the end of the last century by 



