BIOLOGICAL EFFECTS OF THE TIDES 257 



exposure to the sun's rays. The breathing difficulty is got over 

 by a complete cessation of activity at low tide, a kind of 

 hibernation. Where the animal lives high up the shore its 

 periods of inactivity may far exceed its periods of activity. 

 Thus, barnacles may remain dormant for a period equivalent 

 to nineteen-twentieths of their whole existence. The risk of 

 desiccation is countered in many ways. Burrowing in the 

 sand is one of them, hiding under stones or seaweed ano- 

 ther. These are behaviour responses. Sedentary and sluggish 

 animals, incapable of burrowing or hiding, show structural 

 modifications, such as shells and tubes, which aid in the con- 

 servation of moisture. One of the most interesting of these is 

 seen in the common rock-barnacle, the shell of which is closed 

 by accurately fitting valves. At low tide, a bubble of air may 

 be retained between the tips of the valves, and its oxygen is, 

 no doubt, an adjunct to respiration. The clearly perceptible 

 clicking sound heard when one is walking over barnacle- 

 covered rocks is due to the complete shutting of the valves as 

 a protective measure, and consequent rupture of the air- 

 bubble.^ Analogous arrangements are seen in the opercula of 

 Gasteropod molluscs, and so on ; to discuss them all would 

 require more space than we have at our disposal. 



There is little need to return to the question of rhythm ; 

 the flat-worm, Convoluta, has provided us with one example, 

 and there would be no difficulty in finding others. As we have 

 seen, the constant reaction to the tidal rhythm has a profound 

 effect upon the behaviour and other functions, and this effect 

 continues even when the organism is withdrawn from tidal 

 influence. In considering the behaviour of shore forms, there- 

 fore, it is particularly necessary to take this factor into account, 

 or our conclusions may be untrustworthy. 



A survey of the statements which have been put forward in 

 this article seems to justify the conclusion that the tides have 

 not only been of very great importance in moulding the present 

 day life of the seashore, but have also had far-reaching conse- 

 quences to life in general. This pulsing, ever-changing strip 

 of the earth's surface has played a part in life out of all pro- 

 portion to its size. 



1 Darwin, C, A Monograph of the Balanoidea, vol. i., 1854. 



