Fauna of the Garihheaa Sea Sc. 315 



What is especially astonishing- is the infinite variety of 

 zoological forms, which often renders it almost impossible to 

 apply the classifications which have hitherto been regarded as 

 most firmly established. In fact, transition types abomid, and 

 we find numerous intermediate forms between groups that we 

 were accustomed to consider very distinct. Of this I will 

 give some examples. 



The family Paguridse, or the Hermit-Crabs, arranged by 

 zoologists of most authority in the group Anomura, hitherto 

 included only species which, although very numerous, were 

 all very similar to each other, and without any direct con- 

 nexion with the Macrura. The American dredgings have 

 furnished unexpected forms, which unite the Hermit-Crabs 

 with the Thalassinidffi. Such is Pylocheles Agassizii, the 

 abdomen of which, instead of being soft and unsymmetrical 

 like that of the Paguri^ is composed of firm regular rings and 

 terminated by a symmetrical fin. This Crustacean lives in 

 holes, the entrance of which it closes with its claws, which, 

 when they are united by their inner margin, constitute a very 

 perfect operculum. The Mixtopaguri differ less from the 

 Paguri ] for their abdomen, which is more developed on the 

 right than on the left side, is divided into seven distinct and 

 movable joints, the first five of which are imperfectly cal- 

 cified, while the last are large and hardened. In the Ostra- 

 conoti the carapace is entirely coriaceous, and the abdomen so 

 reduced that the female, to hold her eggs, makes use of the legs 

 of the fourth pair, the penultimate joint of which, widened into 

 a palette, forms a sort of tioor underneath the packet of eggs. 

 The Catapaguri establish a passage between the preceding 

 and the Sinrojyaguri : their abdomen is still very small, but 

 twisted and lodged in little shells, the dimensions of which 

 contrast strongly with the size of the carapace and legs, which 

 remain outside. In some of these Crustaceans we also 

 observe curious adaptations to a special mode of life. Thus 

 Eupagurus discoidalisj inhabiting the tubular shell of the 

 Deiitaliaj makes use of one of its clielse as a circular oper- 

 culum perfectly moulded to the orifice of the dwelling which 

 it has to close. The Xylojxiguri also merit our attention : 

 they have never been found except in holes hollowed in frag- 

 ments of wood ; and whether it be a reed, a rush, or some 

 branch, these cavities are always open at both ends ; and the 

 animal does not introduce itself backwards, as the ordinary 

 Paguri do, but penetrates into its lodging directly. When 

 there the chelae always make their appearance at one of the 

 orifices, the other being closed by the extremity of the abdo- 

 men, converted into an opercular buckler. 



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