PARASITISM, ETC., ON THE SEA-SHORE 135 



within tidal limits, and of other forms such as Bticcinum, 

 Turritella, and Nattca, which are found beyond low-tide 

 mark. With the crab is found the Polychaet Nereilepas 

 fucata, a commensal which lives in the top whorl of the 

 shell and probably changes shells with the crab. Upon the 

 shell may occur either of the anemones Sagartia parasitica 

 and Adamsia palliata (the latter species more rarely), colonies 

 of the hydroid Hydractinia echinata, and the sponge Suberites 

 domuncula. Both inside and outside occur the bivalve 

 mollusc Anomia and the tubeworm Pomatoceros ; the 

 Amphipod Podocerophis excavata almost invariably infests 

 the dirt at the bottom of the shell, and with it is not infre- 

 quently the small crab Porcellana longicornis. True parasites 

 are an Isopod of the genus Bopyrus^ which inhabits the 

 branchial chamber and the degenerate Cirripede Peltogaster 

 pagiiri (Jackson, 1913). 



Commensalism. — Strictly speaking, this is an external 

 partnership between two forms of different kinds for mutual 

 benefit. Many cases, however, exist which may be allowed 

 to come under this heading where the advantages resulting 

 from the association are by no means equally divided, and 

 may, in fact, be entirely on one side. From such a state 

 of affairs the step to parasitism is not a long one. Similarly, 

 the step to commensalism from a relationship which is at 

 first merely that of neighbours, is equally short. 



Issel (1918) describes specimens of the fish Serranus as 

 standing sentinel outside the holes or crevices inhabited 

 by Octopus. This is probably a very loose association, the 

 fish feeding upon the remains of the Crustacea devoured by 

 the mollusc. Frequently, one form will take advantage of 

 the shelter afforded by the body of another form, and the 

 two are then usually described as being commensals although 

 the benefits are all on one side. Colonial forms such as 

 sponges and corals offer safe hiding places, and are much 

 utilised by other animals. A common example of this 

 incomplete commensalism is that of the bivalve Modiolaria 

 and the sea-squirt Ascidia mefittda, the mollusc colonising 

 the whole of the " test." Occasionally an association of thi§ 



