THOMPSON: METAMORPHOSES OF HERMIT CRAB. 197 
ment during the development of the abdominal position would be 
the most likely outcome. These considerations make it seem prob¬ 
able that the shell was taken early in the history of the group, and 
so indirectly support that theory which derives the asymmetry pri¬ 
marily from use of shells. 
Another important consideration adds further support to this 
view. At the time when the ancestor or ancestors of the hermit 
crabs began to seek other residences than burrows or crevices, the 
chances would have been favorable for the immediate use of dextrally 
spiral shells. The Pagurid group cannot have originated before the 
late Cretaceous or early Eocene (O.rtmann, : 01) while the Gastro¬ 
poda are geologically a very old group. So we may suppose that 
a supply of mainly dextral shells would have been at hand when the 
hermit crabs besjan to use “ movable burrows ” and the asvmmetry 
might have been affected by the shell at the very outset. This of 
course leaves the sinistral asymmetry of Paguropsis unexplained. 
Thus the whole question falls back on general considerations, and 
on such a basis there is.no reason to abandon our “shell” theory 
despite the lack of positive evidence. The relationships of such 
forms as Mixtopagurus and Paguropsis to the other genera of the 
group are wholly unknown. In fact, the relationships existing 
between the various genera as a whole, are very obscure. The 
group is in all likelihood convergent and not natural, although there 
is reason to believe that most of the genera are genetically related. 
If convergent, the parts of the group may be of different ages, the 
characteristic mode of life being assumed at different times by vari¬ 
ous forms. All this confusion must be cleared away before the 
asymmetry can be thoroughly understood. And this will require 
a vastly more extensive anatomical knowledge than we at present 
possess. The extent of the group and the poor preservation of the 
internal anatomy, particularly in the abdominal region, have hitherto 
turned investigation toward description of species and genera. 
While this is necessary, it usually does not lay enough emphasis upon 
the anatomy as a whole, but is content with a record of the more 
obvious and superficial detail of carapace and limbs. JSow research 
must go farther afield. We need information with respect to the 
finer details of the structure in each species. I believe the clue to the 
origin of the asymmetry and to the phylogeny of this group of Crus¬ 
tacea lies in a study of the internal rather than the external anatomy. 
