i 7 2 ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



MOLLUSCS (MOLLUSCA) AS MESSMATES 



A marine snail (Pleurotoma symbiotes, fig. 1125), living in 

 the deep water of the Indian Ocean, always has its shell more 

 or less encrusted with colonial sea-anemones (Epizoanthus}. Both 

 animals are no doubt benefited, for the mollusc is protected, while 

 the anemones are carried about. 



A number of small Bivalve Molluscs are associated with bur- 

 rowing Sea- Urchins or Crustaceans. Or^e such bivalve (Monta- 

 cuta ferruginosa), native to South Devon, lives in the dwelling 

 which a Heart -Urchin (Echinocardium cordatum) excavates in 

 muddy sand. The circulation of sea -water which takes place 

 within the burrow (see vol. iii, p. 357) ensures 

 a constant supply of food by which the mollusc 

 benefits. In places where the sand is loose 

 and wet the Heart- Urchin is in the habit of 

 coming to the surface, along which it makes 

 its way, but the lodger is not thereby left be- 

 hind, for it spins byssus threads that attach it 

 to its partner. 



A rare little British Bivalve (Lepton squa- 

 c F !f, I " s -- AnIndian . s / ea ; mosum) inhabits the burrow of a prawn -like 



Snail (Pleurotoma symmotes) I 



with commensal Sea - Ane- Crustacean (Upogcbia stellato), and, having an 



mones (Epizoanthus} n i 11 i r -i i 



exceedingly nat shell, does not interfere with the 

 movements of its protector. A similar partnership exists on the 

 coast of Florida between two species related to the preceding, 

 while on the shores of Oregon and California a third association 

 of the sort is more intimate, for here the Lepton attaches itself to 

 the abdomen of the Upogebia. A burrowing Australian prawn 

 (Axius plectorhynchus] harbours two species of a kind of bivalve 

 (Ephippodonta), which is never found elsewhere. The flatness, 

 so necessary to allow of the restless movements of the prawn, 

 is here produced by the valves of the mollusc opening to their 

 fullest extent. This particular prawn appears to be a specialist 

 in the matter of providing lodgings, for four other bivalves (one 

 species of Kellia and three of Mylitta] find a commodious home 

 in its burrow, which also contains an orange -coloured sponge. 

 The last possibly serves as a protection to the crustacean, but the 

 arrangement would appear to be quite one-sided so far as the 

 molluscs are concerned. 



