PAGURIDEA — SYMMETRICAL HERMIT-CRABS 



173 



Besides the ordinary twisted Tagurids 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 Investigator 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 when the animal is withdrawn, 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 diiferent mode of 

 life, and lost, or partially lost 

 those characteristics of ordinary 

 Hermit-crabs which are asso- 

 ciated with life in a spiral shell. 

 These are the Lithodidae and 

 the " Eobber - crab," Birgus 

 latro, of tropical coral islands. 



Although the Eobber-crab 

 and the Lithodidae bear a 

 certain superficial resemblance 

 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 clue 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 



Fig. 118. — Pylocheles miersii, x 1. A, End 

 view of a piece of mangrove or bamboo, 

 tlie opening of wliich is closed by the 

 great chelae (c) of the Pagurid ; B, the 

 animal removed from its house. (After 

 Alcock.) 



' Alcook, loc. cit. ; Borradaile, op. cit. p. 162 ; 



1. p. 



