210 THE IHFLUENCB OF INANIMATE SURKOUNDINGS. 



its hold of the object to which it is clinging, and, in Tpoiait of 

 fact, many Neritinae attached to stones fall off at the slightest 

 touch. There is no doubt that they thus easily escape from 

 the pursuit of their enemies, precisely as many beetles living 

 on leaves elude the search of the entomologist by dropping off 

 them. If such a Neritina were suddenly transferred to a 

 rushing mountain stream, it would undoubtedly soon be crushed 

 by the rolling of the pebbles, if it preserved its habit of falling 

 at an unexpected jar. 



All animals possessing this peculiarity are, therefore, unfitted 

 to live in currents of any force ; and the creeping creatures living 

 in such a situation must be able either to hide in the narrowest 

 cracks and fissures in the rocks, or to bore into the earth or the 

 stone itself, or else must have such clinging powers of the foot 

 as may enable them to resist the shock and pressure of the water 

 by attaching themselves firmly to the rock, and applying the 

 margin of the shell so closely to the surface of the stone that it 

 Ls diflScult or impossible to remove them. This is in fact the 

 case ; all the Neritinas that I found clinging to the surface of 

 the rock in exposed spots in the mountain streams of the 

 Philippines, adhered as closely as possible, as soon as I touched 

 them. But in this respect many Navicellse are still better 

 qualified; they often live in the midst of a dashing torrent, 

 adhering to the stones by suction, so closely that it is difficult 

 to raise them, even with a knife, without injuring the margin of 

 the shell. This is efiected partly by the sucking power of the 

 foot and partly by the form of the shell, which is flattish, 

 conical, and oval, and has an operculum which is smaller than 

 the marginal circumference of the shell. The relatively small 

 size of the mouth is owing to a calcareous plate forming a sort 

 of half-deck across the back of the inside of the shell, aai which 

 may be considered as identical with the inner whorl of other 

 Univalve Shells, while the foot is so broad that it quite covers 

 the under surface of this half-deck, whUe its margin corresponds 

 exactly with the oval of the mantle, and also corresponds with 

 that of the shell. If a Navicella is removed from the spot 

 to which it is clinging — and this is not difficult to do if it is 

 taken by surprise — it can be seen at once that the animal cannot 



