64 THE BIOLOGY OF THE SEA-SHORE 



opercula and similar devices in this connection, it is by no 

 means certain, as Colgan remarks, that the opercula act by 

 perfect exclusion of the fresh water. For how are we to 

 explain the fact that individuals of littoral species which 

 have remained closed for considerable periods when exposed 

 to dry air or immersed in fresh or abnormally salt water, 

 return to activity almost immediately when placed in normal 

 sea-water ? Is the operculum permeable only to water 

 containing a definite concentration of salts ? This seems 

 unlikely. To us, the most feasible explanation seems to be 

 that the mollusc first makes trial by allowing the operculum 

 to relax by a minute fraction. If the result is favourable, 

 the animal relaxes still further. The subject, however, 

 merits further investigation. 



The habit of tube-building among Polychaets may, or 

 may not, have its primary origin in the necessity for avoiding 

 enemies and possibly also wave-shock, but in the case of 

 littoral Polychzets it is certainly of value in preventing 

 desiccation also. Actively moving Polychaets such as the 

 Nereidae have no difficulty in keeping in touch with their 

 proper requirements ; moreover, they are able to burrow. 

 The habit of burrowing is one of the commonest methods 

 of avoiding both desiccation and enemies. It has the 

 additional advantage that it does not necessarily imply 

 cessation of activity. 



The fisherman's lug-worm, Arenicola marina, inhabits a 

 U-shaped burrow of which one end is marked by a casting 

 and the other by a funnel-shaped hole or depression through 

 which the head is probably protruded when the burrow is 

 covered by the rise of the tide (Ashworth, 1904). Low tide 

 for these burrowing animals is not the time of complete 

 stagnation that it is for the sedentary forms. Feeding still 

 goes on to a certain extent, but during spring tides and in the 

 summer months there is probably considerable accumulation 

 of decaying organic matter which, swelled by the excretions 

 of the animals themselves, gives rise to progressively un- 

 favourable conditions which would result in the death of 

 the worms by auto-intoxication if it were not for the return 



