196 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Other anatomical details also fail to throw light on the problem. 
The alterations by which the posterior end of the body has been 
transformed into a hook-like organ point to life in a movable 
residence, bat might have been developed in either a straight or a 
spiral house. The tuberculated areas on the uropods in the more 
symmetrical hermit crabs and on both uropods and posterior thoracic 
feet in the asymmetrical forms, do not even indicate life in a mov¬ 
able residence. They are found in Pylocheles, which lives in the 
cavities of sponges. The reduction of the ventral abdominal artery 
also may have come in either early or late in the development of the 
hermit crab group. This vessel is very weak in some Thalassinids, 
e. g ., Gebia deltura (Bouvier, ’90), and is still present in the 
anterior abdomen of so asymmetrical a Pagurid as Paguristes 
(Bouvier, ’90a). The record might be thus continued with respect 
to other characters and with the same result. Our knowledge of 
the adult anatomy is insufficient to throw light upon the origin of 
the asymmetry, although, as already noted, the anatomy has been 
profoundly modified for the life in spiral shells. 
The metamorphosis is also inconclusive on this point except in one 
particular; and this merely points to an early use of a dextrally 
spiral shell and does not bear upon the beginnings of the asymmetry. 
I refer to the displacement of the right liver to the left of the intes¬ 
tine and mid-line of the abdomen as the livers pass back from the 
thoracic position. That this must be interpreted as palingenetic 
seems to me to be fairly certain. Not only is it natural to extend 
this significance from the liver, shift itself to the displacement, but 
the change also is an essentially transitory phenomenon and it is 
very hard to imagine any cause on other grounds for its introduction 
into the metamorphosis. If the displacement be palingenetic, it will 
go far to support the idea of an early use of shells. The shift of the 
livers in the phylogeny of the group probably occurred very early, for 
these organs have had time to attain a complicated structure since 
reaching the abdominal region. Further, they lie far back in the 
thorax in modern Thalassinids. Now, if these organs were already 
abdominal at the time when the shell was first assumed, it is difficult 
to understand how the compression of the right side of the body 
against the columella of the shell could have caused a displacement 
rather than an atrophy of the right liver. But if they were still 
thoracic or barely abdominal when life in shells first began, displace- 
