No. 4.— THE METAMORPHOSES OE THE HERMIT 
CRAB. 1 
BY MILLETT T. THOMPSON. 
Introduction. 
The group of the Pagurids or hermit crabs lias always attracted 
the attention of carcinologists, not only because of its extent both in 
species and in individuals, but also because of the asymmetry which 
involves nearly every genus and the habit of protecting the soft 
abdomen within hollow objects, typically the shells of Gastropod 
Mollusca. Whether the asymmetry — which is of dextral type 
-except in the genus Paguropsis — owes its origin to this use of 
.shells for residences, since nearly all marine Gastropod shells are 
dextral in coil, cannot be determined with certainty until the phy- 
logeny and relationships of the various genera are better understood. 
But, nevertheless, it is unquestionable that the modifications found 
are in a very great degree correlated with residence in dextrally 
spiral shells. This alone makes the ontogeny of the group an 
-extremely interesting and important subject for study. 
Knowledge of Pagurid ontogeny was in its beginnings practically 
contemporaneous with the discovery of the metamorphosis among 
the higher Crustacea. During the discussion which followed 
Thompson’s assertion (’28) that the supposed genus “Zoea” was 
a larva, Vigors (’30) in a review of Rathke’s study of the develop¬ 
ment of Astacus fluviatilis , which has no metamorphosis, appeared 
skeptical with regard to Thompson’s conclusions. This drew a 
reply from the latter author (Thompson, ’30-’3l), communicating a 
list of fifteen Decapods in which he had observed the young to be 
unlike the parent and “Pagurus” was included in this list. 
The further statement by Thompson five years later, in 1835, that 
Both “ Zoe ” and “Megalope” were larvae and his reiteration that, 
among “Macrura,” Astacus marinus, Pagurus, and other forms 
underwent a metamorphosis, greatly stimulated research on crusta- 
1 From the Anatomical laboratory, Brown university, and the laboratory of 
the U. S. fish commission, Woods Hole, Mass. 
