VI PAGURIDEA—-SYMMETRICAL HERMIT-CRABS 173 
Besides the ordinary twisted Pagurids which inhabit Gas- 
teropod shells, there are a few which preserve the symmetry of 
the body. The interesting Pylocheles miersii' (Fig. 118), 
taken by the Jnvestigator in the Andaman Sea at 185 fathoms, 
inhabits pieces of bamboo; it is perfectly symmetrical, with 
well-developed pleopods | and 
symmetrical chelae, which, 
when the animal is withdrawn, 
completely shut up the entrance 
to its house (Fig. 118, A). 
It is doubtful whether this 
animal ever inhabited a spiral 
shell or not in its past history ; 
but there is no doubt that 
a number of peculiar crabs, 
which caused the older sys- 
tematists much trouble, are 
Pagurids, derived from asym- 
metrical shell- haunting an- 
cestors that have secondarily 
taken to a different mode of 
life, and lost, or partially lost 
those characteristics of ordinary To 
Hermit-crabs which are asso- . 
ciated with life in a spiral shell. 
These are the Lithodidae and 
the “ Robber - crab,” Birgus 
latro, of tropical coral islands. — pyq, 118.—Pylocheles miersii, x 1. A, End 
Although the Robber-crab yer of ple of mmngors o: bane, 
and the Lithodidae bear a great chelae (c) of the Pagurid; B, the 
certain superficial resemblance ae oes Ouse dante 
to one another in that they 
lead a free existence, and have reacquired to a great extent their 
symmetry, yet it is clear that they have been independently 
derived from different groups of asymmetrical Hermit-crabs, and 
that their resemblance to one another is due to convergence. 
Birgus latro (Fig. 119), a gigantic crab, frequently over a 
foot in length, lives on land, and inhabits the coasts of coral 
islands in the Indian and Pacific Oceans where cocoa-nut trees 
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At 
An 
v 
1 Alcock, loc, cit. ; Borradaile, op. cit. p. 162; i. p. 64. 
