168 e A HISTORY OF RECENT CRUSTACEA 
the Epizoanthus americanus fixes itself. First a single 
polyp finds lodgment, and as its basal membrane spreads 
over the shell, buds arise from it forming fresh polyps, and 
gradually this same membrane absorbs the shell though 
retaining its spiral shape. In this absorption there is a 
great advantage to the hermit, because as it grows its in- 
crease of bulk still finds room in the yielding polyp-mass, 
without any necessity arising for a change of domicile. In 
the case of Catapaqurus Sharrert, A. Milne-Edwards (see 
Plate IV.), there is sometimes a triple alliance, for over a 
colony of Hpizoanthus americanus there settles itself the 
single polyp Adamsia sociabilis. The numerous specimens 
of Parapagurus pilosimanus that were taken in depths 
reaching to six or seven hundred fathoms were found only 
in colonies of Hpizoanthus paguriphilus, but those that were 
taken at much greater depths ‘ were either in a very different 
species of Hpizoanthus, in naked gastropod shells, or in an 
actinian closely resembling, if not identical with, Urticina 
consors, Verrill. Dr. Carl Aurivillius has recently pub- 
lished very interesting observations on the ‘Symbiosis’ or 
living together of Hydroids, Sponges, and Pagurids. Hy- 
dractinia echinata is frequently found coating the outside 
and inside of various shells that are occupied by Hermit- 
crabs. The mode of growth is such that the hydroid not 
only repairs in effect the damaged mouth of a shell, but 
also frequently extends its boundaries. This is especially 
the case in districts where shells are few, and where the 
growing Pagurid might be put to much inconvenience to 
find a larger lodging. By the extension of the hydroid 
colony, which sometimes gives a quite monstrous appear- 
ance to the shell, the hermit is saved the trouble of making 
any change of abode. The hydroids are saved from the 
danger and damage they would be exposed to from the 
rolling about of an empty shell. On the inside of it they 
do not develop any of the nutritive polyps which might 
incommode the hermit and also suffer injury from its 
movements, but they line the interior with a network, to 
the satisfactory smoothness of which the hermit itself con- 
tributes. It might be supposed that this was effected by 
