THE SEA SHORE 29 



all be plant feeders and live on weed, they may all be 

 attached forms and live fastened to rocks, or be burrowers 

 into mud or sand, or they may be carnivorous beasts which 

 prey upon the animals composing one of these communities 

 to which they thereby become attached. The most 

 intimate form of association is that of parasite and host (as 

 we call the animal which " entertains " the parasite), or the 

 more equal type of intimate union known as symbiosis, 

 some account of which is given on page 213. Lastly, 

 animals, without being actually united with one another, 

 may always live together, one perhaps upon the other, a 

 condition called commensalism, because the two feed in 

 common and also frequently assist one another. Examples 

 of this are given later in this chapter. 



We can recognize a series of fairly definite zones as we 

 pass down from high -water mark over the shore at dead low 

 water. On rocky shores we can distinguish these zones 

 by the different levels at which the principal sea weeds 

 grow. The different coloured weeds have a perfectly 

 definite arrangement, the green ones generally growing in 

 pools near high-water mark or even above it where sea 

 water only occasionally penetrates and the water is largely 

 fresh ; the brown weeds are especially common between 

 tide-marks, as no one who has ever walked on a rocky 

 shore can fail to have realized, while below them come the 

 red weeds. These are usually found between tide-marks at 

 the bottom of the deepei rock pools or when the spring 

 tide uncovers an exceptionally large area, but are commonest 

 in shallow water off a rocky shore. 



Above high-water mark, except at the highest spring 

 tides, there is an area of mixed salt and fresh water — 

 brackish is the term used to describe it — which, if the ground 

 be marshy, is usually characterized by the presence of the 

 little salt-wort, a tiny plant (not a sea weed) which has 



