REPORT ON THE ANOMURA. 213 



might be expected, for it is knowu that the supply of oxygeu is diminished at great 

 depths, and in many other deep-sea animals the respiratory surface is apparently increased 

 with the object as it were of counterbalancing this diminution in the supply. I am, 

 therefore, led to the conclusion that this character is an ancestral one, and that certain 

 at least of the Pagurodea are the descendents of Macrura in which the gills were tricho- 

 branchiate. This view is partly supported by the fact that in Pylocheles, A. ]\Iilnc- 

 Edwards, a deep-water genus which is represented by a single species in the collection, 

 and retains to a remarkable extent the primitive Macruran characteristics, the gills 

 have essentially this structure. 



The other structural modifications met with in the deep-water Pagurids are 

 comparatively few and unimportant. The eyes as a rule are slightly reduced in size, 

 but in all cases they are pigmented and apparently functional, a fact which would lead 

 us to suppose that the Pagurodea have spread more recently into deep water than the 

 Galathodea. The most characteristic genus is Parapagurus, S. I. Smith, the species of 

 which appear to live invariably in a state of commensalism with an Anemone, which 

 exerts a solvent action on the shell originally taken possession of by the Hermit, so that 

 in many cases the latter is merely protected by its clinging messmate. I have failed to 

 detect a single instance in which a shell had not apparently been present at one time. 

 In all the females which I have examined the genital opening of the right side is 

 apparently absent, a peculiai'ity which doubtless coincides with a deficiency in the 

 internal reproductive organs, though I have been unable to satisfy myself on this point. 

 One of the species, Parapagurus ahyssorum, A. Milne-Edwards, appears to be almost 

 universally distributed at great depths, afi'ording a noticeable instance of the greatly 

 extended distribution enjoyed by many abyssal species. 



The Pagurid which I have described under the name of Tylaspis, and which occurred 

 at a depth of more than 2000 fathoms, is remarkable chiefly for the fact that its gills 

 have essentially the ordinary phyllobranchiate structure, though the characters of the 

 carapace and abdomen are also sufliciently marked. Paguropsis, another of the new 

 genera, also presents several features of extreme interest. It is distinguished by the 

 large size of its eyes, and by two characters in which it stands apart from all other 

 Pagurids, viz., the last two pairs of thoracic legs are subdorsal in position, and the 

 unpaired abdominal appendages occur on the right side. 



It is, however, among the Galatheidea that the largest proportion of deep-water forms 

 is found. 



The Porcellanodea, or more highly specialised Galatheids, are almost confined to 

 shallow water, but a new species {PorceUcma robertsoni) was taken in the West Indies 

 at a depth of 390 fathoms, which in all probability marks approximately the extreme 

 vertical limit of the group. 



The Galathodea, on the other hand, are found abundantly to a depth of about 2000 



