SIPHON-WORMS AS MESSMATES 183 
mensalism as exists between the Aspzdosiphon and the coral is 
an interesting one. Commensalism is usually looked upon as 
conferring some mutual advantage on the contracting parties, 
and one or the other of these usually seeks the other out. But 
in the case in question the mutual advantage is far to seek. It 
can hardly help the coral to have a large proportion of its base 
burrowed by a spacious canal, but the fact that the Gephyrean 
pulls the otherwise immovable coral about may be, and probably 
is, an advantage to the Ccelenterate. On the other hand, the 
Gephyrean gains protection and a home more spacious than the 
Gastropod shell affords. The Aspzdostphon can hardly find or 
attract the larval coral to come to rest on its borrowed shell, and 
it is unlikely that the larva is especially on the outlook for such 
shells as are inhabited by Gephyrea. It seems more probable 
that the Aspzdosiphon may select for its home a Mollusc shell 
which already bears a young coral, but the whole matter seems 
to demand more careful study. It is certainly remarkable that 
three distinct genera of coral, each with but one species, should 
be inhabited by three distinct species of Aspzdostphon, and that 
neither commensal has hitherto been found apart from the other.” 
In some cases, at least, there would appear to be a third partner 
in the concern, for numbers of a kind of minute bivalve mollusc 
were found closely attached to the outside of the siphon-worms. 
Regarding them Shipley remarks:—‘‘These were so closely 
adpressed to the skin of the Aspzdoszphon as to indent it, appear- 
ing as little pearls set in a matrix. The advantage they obtained 
by taking up such a position is not very evident, but there they 
were, and as far as one could judge they were, until Professor 
-Herdman dropped them into his collecting-jar, flourishing.” 
