PLANT AND ANIMAL ASSOCIATIONS 53 



Jetsam Association. — A distinct and interesting animal 

 association whicli must not escape our notice is that occurring 

 among the jetsam deposited by the waters at high-tide mark. 

 The most important form is undoubtedly the sand-hopper 

 Talttrus locusta, which performs useful work as a scavenger 

 by devouring plant remains. Other forms with the same 

 food are Staphylinid beetles and Enchytrasid worms. 

 Feeding on decaying animal remains are the flesh-flies 

 ^Sarcophagus) and carrion beetles (Necrophorus). Associated 

 with these again is a predaceous fauna consisting of spiders, 

 tiger-beetles, etc. (Davenport, 1903), and finally birds 

 (crows, jackdaws) can generally be seen picking over the 

 heaps and taking their choice of the foregoing. 



THE STRUGGLE FOR EXISTENCE AMONG SHORE ANIMALS 



A few remarks may not be out of place here regarding 

 the struggle for existence among the inhabitants of the shore. 



This struggle falls into two categories : (i) against 

 physical conditions which have already been discussed, and 

 will be treated further in the next chapter ; and (2) the 

 interaction and competition of species with species, and 

 individual with individual. 



Effect 0! Currents on Dispersal 0! Shore Forms. — 

 In this connection there arises a factor that has not yet been 

 discussed, viz. that of currents as agents in the dispersal of 

 shore forms, both plant and animal. Many of these are, 

 in their early stages, either free-swimming or floating, and 

 are then carried to and fro, and hither and thither, by the 

 tides, and are thus temporarily members of the Plankton. 

 They are thus, for a time, removed from some of the typical 

 dangers of the tidal zone (though exposed to some others). 

 Many of these forms return to settle near their point of 

 origin, while many others are, naturally, carried far by tidal 

 currents, and when ready to assume the sessile, crawling, 

 or burrowing stages of later life, may have arrived in a region 

 either more or less favourable than that from which they 

 were transported. Thus dispersal commences. Those 



