172 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
MOLLUSCS (Mo.tiusca) 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 (Zf7zoanthus). 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. One such bivalve (J/oxéa- 
cuta ferruginosa), native to South Devon, lives in the dwelling 
which a Heart-Urchin (Zchinocardium cordatum) excavates in 
muddy sand. The circulation of sea-water which takes place 
within the burrow (see vol. ii, 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 (Lefton sgua- 
See shee tae mosum) inhabits the burrow of a prawn-like 
with Commensal Sea-Ave: Crustacean ( Upopedta stead), and, having an 
mones (E£pizoanthus E a 
exceedingly flat 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 
(Axis plectorhynchus) harbours two species of a kind of bivalve 
(LZ phippodonta), 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 Ae//“a and three of J/;/¢t/a) 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. 
