METHODS OF ATTACK AND DEFENCE 113 



rapidly, the time during which they are under construction 

 being probably one of danger to the animal. According to 

 Bauer, similar phenomena, which possibly have their 

 biological explanation in the relations between foes, are : 

 (i) the reinforcement of the whole of the last turn in the 

 shells of Conus and Cyprcea ; (2) the strong calcareous 

 operculum (sometimes with spinous processes) of numerous 

 unrelated forms ; and (3) the frequency of the disk-like 

 shape in unrelated groups, which is a protection not only 

 against wave-shock, as in Patella^ but against marauders 

 as well. 



The observations of Bauer are of considerable interest, 

 and we have accordingly dealt with them at some length, 

 but it would be unwise to regard the various shell modifica- 

 tions which he discusses as being due exclusively to the 

 conflict between foes, though this is no doubt an important 

 factor. Some writers have seen in Gasteropod spines an 

 adaptation to life in the sand, e.g. Hunt (1885) ; others, 

 again, regard them as the expression of some internal 

 factor such as growth or age (Beecher, 1901). It is for the 

 ecologist to unravel these various factors, to determine the 

 manner in which they interact, and to estimate their relative 

 share in the production of a particular form. 



Retrospect. — Competition on the shore is extraordinarily 

 keen, and the methods of attack and defence are corre- 

 spondingly varied. The dense crowding of this area 

 and of its continuation, the shallow sea, gives a clue to 

 the occurrence of special organs such as the pedicellariae 

 of Echinoderms and the avicularia of Polyzoa which serve 

 to keep the body free of encrusting organisms. This 

 encrustment of certain species by other sedentary forms 

 may be of trivial importance or it may be such as to 

 cause considerable inconvenience. On the other hand, it 

 may be brought under control and turned to advantage 

 Apart from distinct organs of offence and defence we are 

 justified in speaking of " tactics " such as hiding, masking, 

 protective coloration, warning coloration, mimicry, etc., 



I 



