r 
1891.] A Commensal Annelid. 27 
session of by the hermit or else became buried or washed away. 
In no case could an Annelid be found in a shell containing the 
original proprietor,—the Ilyanassa. 
A somewhat similar case of an Annelid occurring in the dwell- 
ing of a hermit crab was long ago noticed by Quatrefages, but 
there the Annelid, a Nereis, may possibly have taken up only 
temporary quarters within the shell. This, however, seems not 
to be the case with the Nereis described by Wirén in 1888, 
which, it would appear, has been modified by its well-acquired 
habit of living inside such shells. The Polydora, however, 
does not merely live within the pre-formed cavity of the Gaster- 
opod shell, as the Nereis would, but, by boring a tunnel in the 
columella, and by partly filling up some of the spiral cavity of the 
shell with a calcareous tube of its own manufacture, reconstructs 
the shell to fit its own needs. 
The general character of the dwelling made by the Annelid 
within the Gasteropod shell may be inferred from a view of the 
shell cut lengthwise into halves, as in Fig. 1. The external 
opening of the tunnel is seen on the inner lip of the mouth of the 
shell as a conspicuous rounded hole, which leads by a long passage 
inside the columella up to the spiral part of the shell. Here 
the tunnel opens out again into the apical chamber of the shell 
by the round hole seen in the figure. The inner opening, how- 
ever, is continuous with a calcareous tube built into the chamber 
of the shell in such a way as to completely block it and prevent 
the posterior end of the crab, or any other object, from pressing 
up into the smallest terminal spirals. This tube is bent or coiled 
in various ways, but may present, as in the figure, a cross-section 
suggesting that of a Mammalian cochlea. Made of a calcareous, 
cement-like mass, it may be the debris of the bored-out tunnel, 
but is more probably a special calcareous secretion of the Annelid 
such as some of its relatives are known to make. 
This description applies only to certain cases, since many 
irregularities are observed both in the calcareous tube and in the 
tunnel, and some shells present external openings near the apex 
and apparently communicating with the Annelid’s dwelling. 
Only one adult Annelid is found in each shell, and this may be 
z 
