62 THE BIOLOGY OF THE SEA-SHORE 



pressing need of the non-sessile animal is to keep in touch 

 with moisture, and this it may achieve by burrowing, by 

 hiding under stones, in crevices, under seaweed, etc. 



Methods o! resisting Desiccation. — Among sessile 

 animals, as we have hinted above, there occur numerous 

 devices for the retention of moisture. A good example of 

 these is afforded by the acorn barnacles, the commonest 

 animals of the sea-shore, in which the limy cup surrounding 

 the animal is covered by four plates forming an accurately 

 fitting valvular roof. A bubble of air is sometimes imprisoned 

 by the tips of the valves at low-tide and serves for breathing. 

 It is the more complete closing of these valves and the 

 consequent rupture of the air-bubble when barnacles are 

 disturbed which gives rise to the characteristic clicking sound 

 familiar to every one who has walked on barnacle-covered 

 rocks. 



Though a barnacle can only expand and obtain food 

 when covered with water, it is able to live so far above 

 ordinary high-water mark as to remain dry for days at a time, 

 amounting, according to Vaillant (quoted by Herdman, 1892), 

 on an average, to ^ths or |^ths of its life. The same worker 

 has determined by experiment that a barnacle can live out 

 of water for at least forty-four days. This capacity to 

 remain for long periods in a condition of suspended activity 

 is a marked feature of sedentary shore animals, and the 

 devices which make it possible are clearly of an adaptive 

 nature. One may notice, in passing, that these periods of 

 suspended activity correspond biologically to the periods 

 of aestivation or hibernation among land animals, the only 

 difference of importance being that the periods of activity 

 or quiescence on the shore are of very much shorter duration 

 and succeed one another at much shorter intervals. 



The opercula of Gasteropod molluscs, horny skin 

 products formed from groups of cells situated on the upper 

 surface of the foot, serve a similar purpose to the valves of 

 barnacles. In many Polychaet worms {e.g. Serpulidae) one 

 (sometimes more) of the gill filaments is enlarged terminally 

 to act as a stopper, which closes the mouth of the tube 



