THOMPSON: METAMORPHOSES OF HERMIT CRAB. 195 
The Asymmetry. 
Although it is evident that the structural modifications possessed 
by the majority of the hermit crabs are on the whole in closest 
correlation with the use of dextrally spiral shells as residences, there 
are two genera in the group which suggest that the asymmetry in 
its beginnings was perhaps not connected with this mode of life. 
Mixtopagurus certainly might be regarded as pointing to an origin 
for the asymmetry prior to, or outside of the use of a coiled shell, as 
it combines a slight dextral twist with residence in holes in bits of 
wood. The sinistral asymmetry of Henderson’s genus Paguropsis 
also seems difficult of explanation on any theory which would derive 
the asymmetry primarily from the use of shells. For the right- 
handed spiral has always been the predominant type of coil among 
the marine Gastropod Mollusca. The point of view with regard to 
these genera remains unchanged whether the Pagurids be considered 
as a natural or a convergent group. 
This question of the origin of the asymmetry seems to me to be 
insoluble at the present day. Detail of the adult anatomy is as yet 
very scanty and for those genera about which most is known it does 
not bear closely on the problem. Probably a large number of the 
anatomical peculiarities of such a typically asymmetrical genus as 
Eupagurus, for example, may be looked upon as inherited; similar 
structures being found among the Thalassinidea, the modern repre¬ 
sentatives of the stock from which the hermit crabs were in all 
likelihood derived (Ortmann, :01). For example, the weak integu¬ 
ment of the abdomen finds its counterpart among the members of the 
latter group. These live in burrows or crevices and probably their 
ancestors had similar habits. So the Pagurids might have attained 
a much weakened integument before they began to exchange 
stationary for movable “ burrows.” There is a curious resemblance 
between the degenerative details of the abdominal muscles of the 
Eupagurids and those of Gebia. Unfortunately, knowledge of the 
abdominal musculature of more generalized Thalassinids than Gebia 
is wanting. And even if the details shall be found to agree in the 
two groups, such a reduction of parts may be merely the usual out¬ 
come where the abdomen is little used by a burrowing species and 
not necessarily peculiar to the Pagurids, nor inherited from their 
Thalassinoid ancestors. 
