PARASITISM, ETC., ON THE SEA-SHORE 133 



extension of the area in the vertical as well as in the hori- 

 zontal sense — all of which mean that a small area may pro- 

 vide foothold for a great variety of creatures — and, finally, 

 the influence of wave-shock in causing frequent displace- 

 ments on the part of the animal inhabitants. All these 

 features seem to make it probable that the needs and the 

 opportunities of forming close relationships are more 

 pressing and numerous on the shore than elsewhere. 



Stages in the Development of Intimate Associations 

 (Commensalism, Parasitism, and Symbiosis) among Shore 

 Animals. — There are many cases on the shore to illustrate 

 how an intimate association between two forms of different 

 kinds may have arisen. For instance, in shallow coastal 

 waters algal spores settle upon every available surface, and 

 it is quite usual to find various small algae growing upon 

 many shore animals ; on the shells of the sluggish limpet, 

 for example, or sometimes on the slowly moving periwinkles 

 and similar forms, while trailing tufts of Enteromorpha fre- 

 quently mark the position of cockles in the sand. Slow- 

 moving Crustacea like Inachiis, which live among algae and 

 hydroids, generally carry a share of these organisms on their 

 backs, and other Crustacea frequently carry small growing 

 plants. These plants are usually without significance, 

 although some of them may be fortuitously of advantage in 

 concealing the individual to which they are attached. In 

 certain cases (spider-crabs, etc.) they may have been 

 placed on the carapace by the Crustacean. Whether it is 

 legitimate to say that this is done on purpose is a matter of 

 opinion (cf. p. 245). It is usual to find the small shore urchin 

 Echinus miliaris with, a number of various algal fronds attached 

 to its spines. A Pleurococcus-Hke alga has been described 

 (Mortensen and Rosenvinge, 1910) as attacking brittle-stars. 

 In this case the colonies of algae appear first on the back of 

 the animal beneath the epidermis. Subsequently they may 

 destroy the connective tissue, lay bare the food canal and 

 cause death. 



A very interesting and instructive case is that of Odostomia 

 rissoideSy which both illustrates a stage in the development 



