Medical Officers of the Army of India.

37

accomplished and successful naturalist Dr. A Willey, who has recently been work-
ing in the seas of the East Indian Archipelago. Dr. Willey (see Borradaile,
P. Z. S. 1899, page 938) saw, in the month of January, at one of the islands
of the Loyalty Group, hatching zoæas being washed off a female Birgus
into the sea. The month of January is said, by residents at Port Blair, to be
the breeding season of Birgus at South Sentinel.

I.—Illustrations of Commensalism from the R. I. M. S.
" Investigator."

       The term " commensalism " has been used with a certain amount of laxity
to cover everything, from the almost chance occurrence of an Amphipod crusta-
cean in the crevice of a sponge, to the fixed and constant association for mutual
benefit that subsists between, say, the hermit-crab Parapagurus pilosimanus
and the sea-anemone Epizoanthus.

       This looseness of language is due to a certain elasticity of the subject. When
two non-parasitic animals are found in company, though we may not be able to
discover any bond between them, yet we are inclined to include the occurrence in
our records on account of its potential importance, for out of such seeming
chance unions all admitted instances of commensalism must have had their
origin.

       Though we are not in a position to insist on any rigid definition of the term,
I shall, in the following selections from my experience in these seas, lay stress
only upon cases in which both of the commensals in any given partnership appear
to be adapted to one another, or in which one of them appears to have undergone
some adaptation to the other without injuriously affecting that other.

       References to the general subject of Commensalism: VAN BENEDEN, Surle Commen-
salisme dans le Règne Animal, Brussells, 1869 (Bull. Acad. Roy. Belgique, Vol. XXVIII),
and Animal Parasites and Messmates (International Scientific Series )

       In BOUVIER, Bull. Soc. Philomath. Paris (8) V. 1892-93, pp. 143-149, a bibliography
containing some valuable references will be found.

       I have not seen the paper by O. HERTWIG, entitled Die Symbiose im Thierreich.

1. Commensalism among Sponges.

       Many kinds of non-parasitic marine animals find a convenient shelter within
the cavities and tissues of sponges and Zoophytes, but we are here concerned
only with cases in which it can be shown, or reasonably believed, that the associ-
ation has gone beyond the casual stage.