188 THE INFLUENCE OF INANIMATE SUKIIOUNDINGS. 



family o{ Orchestidce (fig. 54) live exclusively on land, although 

 they have the gills proper to all aqviatic species. In the summer 

 of 1876 I found in Minorca an enormous number of individuals 

 of one species '^ under large stones in the perfectly dry bed of 

 a stream, during the driest season of the year ; and in the 

 islands of the Indian Archipelago they are often quite as fre- 

 quent as the Land-leeches in damp and constantly shady woods. 

 Various species of Neritina *^ frequently occur on dry land 

 far from any water ; other species live constantly or during the 

 chief part of the year high up on trees in mangrove swamps — 

 groups of Neritina duhia and N. ziczac. 



In most of the cases here adduced, the organisation of 

 the animal appears, so far as we know, to be entirely that of 

 a creature living and breathing in water, or only very slightly 



Fia. Si.— Talili us mllcUor. 



modified. The Orchestidae, Nemertidse, Snails, and Leeches show 

 not the smallest difference from their nearest allies living in 

 v/ater ; in the land Planarians, however, a creeping surface has 

 developed on the under side, vihich acts physiologically in the 

 same way as the foot of the land snails, and which is not found 

 in Planarians living in water. But undoubtedly there are, 

 among Fishes and Crabs (Brachyura) for instance, many forms 

 which constantly, or only occasionally, live on land in damp 

 spots, and have undergone a more or less considerable transfor- 

 mation, particularly in their organs of respiration. As these 

 cases are of more general significance, we will investigate them 

 somewhat more in detail. 



III. The accommodation of water-breathers to breathing 

 air. — Fish, as is well known, breathe through their gills, which, 

 being set at the sides of the head and covered by the operculum, 



