THE INSURGENCE OF LIFE 145 



seems to be peculiarly safe (except from man, wlio some- 

 times taps it), there are many desert plants whose stores 

 form the only water supply of not a few of the desert 

 animals. 



Rock -Borers. — Life's characteristic filling of every 

 niche leads to extraordinary modes of life. Instead of 

 seeking a hfe of ease, many animals attempt what 

 seems impossible, and achieve it. Take the simple case 

 of boring bivalves, like the Pholads, which work their way 

 into hard rock. According to one theory, the boring is at 

 least partly due to an acid secretion ; according to another 

 view it is mainly accomphshed by mechanical means. Miss 

 B. Lindsay made a very careful study of Zirphwa (Pholas) 

 crispata and Saxicava rugosa at St. Andrews, and came to 

 the conclusion that the boring is in these cases entirely 

 mechanical. The Pholas works in two ways — sucking and 

 scraping. ' It might be described as a combination of a 

 nutmeg-grater and a vacuum-cleaner '. The foot is 

 extruded ; a wide gap appears between the foot and the 

 mantle ; the mantle becomes fully extruded, and then 

 rotatory movements begin. An interesting detail is that 

 the shells consist of aragonite, which is harder than the 

 usual calcite, and this must help a little in the process of 

 boring, which remains, however, when all is said, a very 

 remarkable performance. 



Climbing Fishes. — There is a well-known tropical fish, 

 Periophthahnus, which, like its relative Boleophthahnus, 

 spends hour after hour out of water, squatting on the mud 

 by the sides of the estuaries, or even chmbing up on the 

 roots of the mangrove trees. But such climbing powers as 

 Periophthahnus possesses are far surpassed by those of a 

 catfish, Arges marmoratus, which lives in the torrential 



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