THE GENERAL CHARACTERISTICS OF SHORE ANIMALS. 5 



which is uncovered only at the lowest springs, we find a 

 complete series of gradations in regard to exposure to air. 

 The periwinkles mentioned are really under water only for 

 a brief period daily, during perhaps a few days every six 

 months. Then we may have other forms which are covered 

 by water only for a short time at spring tides, and so on 

 down to the animals which are wwcovered only for a brief 

 period during the very lowest spring?. But, as all seafaring 

 people know, the times and heights of the tides as indicated 

 in the calculated tables are in many localities liable to 

 considerable variation on account of winds and storms, so 

 that one must? beware of ascribing too great constancy to 

 tidal movements. All the animals which belong to the 

 shore area, with a few exceptions which need not concern 

 us here, breathe air dissolved in water, so that the fact that 

 they are periodically exposed to the action of the atmosphere, 

 necessitates special means of protection for the delicate 

 breathing organs. The amount of protection required must 

 necessarily vary with the amount of exposure. 



The risk of injury to the breathing organs is not the only 

 danger to which the ebb of the tide exposes shore animals, 

 for the removal of the water makes feeding impossible to 

 not a few of them, and it also exposes them to variations of 

 temperature the frosts of winter and the sun of summer 

 and to the keen eyes of the birds which flock to the rocks 

 as the tide ebbs. Furthermore, as the water returns its 

 waves batter furiously against the rocks and their denizens, 

 so that these have manifold dangers to guard against. 



Among the general characters of shore animals we should 

 thus expect to find that they usually possess some means of 

 protection against the risk of exposure to the atmc sphere, 

 with the correlated risks of freezing or drying up, and 

 also against the force of the waves, which tend to tear 

 them away from their rocky homes. In point of fact, we 

 do find that shore animals show many adaptations to these 

 conditions of shore life. In the first place, very many of 

 them possess shells into which the animal can retire, and 

 which serve to protect it against variations of temperature 

 and the risk of drying up. Shells are especially character- 

 istic of the greater number of the Mollusca, or " shellfish " 

 par excellence, but are also possessed by not a few other 



