( 6 ) 



the difference between Bir^us and Parapagurus is extrennely slight, yet the one 

 is (except in its larval stages) a land animal, while the other spends its life in 

 the abysses of the ocean. 



1 he typical Paguroid lives in a cast-off Gastropod shell, for the protec- 

 tion of its soft abdomen Shells with a dextral twist being the rule, and 

 those with a sinistral twist being the exception, it follows that a right-hand 

 shell is practically always used and moulds the abdomen, giving it a per- 

 manent curl to the right and causing (by pressure) an atrophy of its appen- 

 dages of the right side. 



This right-sided asymmetry is stamped upon the group ; for not only does it, according 

 to Milne Edwards and Bouvier, persist in those rare abnormal individuals that adapt themselves 

 to a sinistral shell ; but also it is equally manifest in at least the females of certain forms 

 (such as the Lithod/da-, Cancellus, Tylaspis, Os raconotus) that are either quite free throughout 

 life, or at any rate do not live in a spiral shell. 



The explanation of the matter is not quite so simple as it looks, and those (if there be any) 

 who are inclined to regard it as a case of inheritance of an acquired character, have still to 

 explain how it is that in Paguropsis (a hermit-crab, closely related to Pagnriitei and sprung 

 from asymmetrical ancestors, which does not use a shell and has regained a secondary sym- 

 metry) the abdominal appendages are atrophied in some individuals on the right side and in 

 other individuals on the left. 



As the hermit-crab grows, it, as a rule, shifts periodically into a larger 

 habitation ; but where, as in the depths of the ocean, the supply of suitable 

 shells is limited, other adaptations to growth are possible. 



it is very common for the shell inhabited by a hermit-crab to also 

 accommodate out-door tenants, such as zoophytes, cirripedes, sponges, tubi- 

 colous annelids, etc. Sometimes, no doubt, the association is merely acci- 

 dental ; but often it is a true commensalism, or definite alliance for mutual 

 benefit, which may even go so far as to lead to mutual adaptations of 

 structure. 



Perhaps the best known of these permanent alliances of hermit-crabs 

 and zoophytes is that of Eupaouras pndeauxii, an inhabitant of European 

 seas, with the sea-anemone Adnmsia palliata. In this case the sea-anemone 

 is merely attached to the hermit's shell, but when the hermit changes its 

 shell it takes its partner with it : the union, although it " bends with the 

 remover to remove," is yet a fixed one. 



With Parapaoiirns pilosimanns — one of the most characteristic crusta- 

 ceans of the deep-sea — the alliance with a compound zoophyte of the genus 

 Epizoanthiis is, literally, fixed ; for the two animals begin life, one as an 

 indoor, the other as an outdoor tenant of the same shell, and as they grow 

 the shell is gradually absorbed until at last the hermit-crab comes to lie in 

 a tunnel of the common substance of the Epizoanthus-colony. 



