JOINTED-LIMBED ANIMALS AS MESSMATES 181 
together in such a fashion that each one assists the other in some 
definite way, while doing it no manner of harm, they are termed 
commensals or messmates. For instance, when a hermit-crab and 
a sea-anemone live together, the hermit-crab, being by nature a 
very ill-clad and vulnerable animal, acquires by the partnership 
a thick and easily-adjustable greatcoat, while the sea-anemone, 
being by nature a hopeless lump of an animal, dependent on 
chance currents for its 
food and oxygen, ac- 
quires an engine and 
intelligent engine-driver 
all in one, which are 
always carrying it in the 
way of the necessaries 
of life; and yet with 
this mutual assistance 
there goes absolute in- 
dependence in all other 
respects, such as mis- 
tresses and_ servants, 
who would both be 
none the worse for a 
little knowledge of the 
principles of zoology, 
never dreamt of.” 
Certain crabs have 
sponges as messmates, 
the mutual advantages" *°— MGW: Tice aneneca(emecmee 
being much the same 
as before, it being remembered that sponges are usually avoided 
by predaceous creatures which appreciate the flavour of crus- 
taceans. In the members of one family of crabs (Dromide) the 
last pair of legs are modified in relation to the commensal habit, 
being small, with more or less hook-like tips, and having shifted 
somewhat towards the upper side of the body. They are used 
to hold a sponge or some other passive messmate that serves 
as a sort of living cape, promoting concealment and protection. 
In the common Sponge-Crab (Dromia vulgaris), as the popular 
name indicates, this companion is a sponge. So also in a little 
species (Cryptodromia pileifera, fig. 1130) from the coral-reefs 
