Medical Officers of the Army of India.

41

case of Adamsia —the sea-anemone lives on quite happily alone if its hermit-crab
partner dies.

      Another similarly doubtful case is that of the sea-anemone which,in these
seas, almost always lives with the Oxystome crab Dorippe jacchino. For protection
this crab commonly holds a dead lamellibranch shell over its back, and to the
upper surface of this shell a sea-anemone is usually attached. It looks, however,
as if there were some special purpose in this association, because I have never
observed a sea-anemone keeping company with the Dromioid crab Conchoecetes
artificiosus although this crab has the same habit of roofing itself over with a
dead lamellibranch shell. Very possibly also the sea-anemone which fixes itself
on the carapace of large adults of the Oxyrhynch crabs Doclea ovis and Doclea
canalifera is a true commensal species.

      The alliances that exist between the little corals Heterocyathus and
Heteropsammia, on the one hand, and certain species of Sipunculoid worms on the
other hand, have been frequently noticed in zoological literature. The worm oc-
cupies a tunnel in the base of the living coral, from which it can, on occasion,
protrude the anterior part of its body—repaying the coral for this accommodation
by acting as a locomotive. Three species of Heterocyathus and three of
Heteropsammia are found in these seas, and when serving on board the " Investi-
gator," I once, for a short time, kept some specimens of Heteropsammia aphroāes
alive to watch how the commensal worms dragged their coral partners about.

      This, as several authors have remarked, is an undoubted instance of com-
mensalism, for both parties are equally benefited by their union.

      One of the happiest instances of commensalism observed on board the
" Investigator" is that exhibited by the gymnoblastic Hydroid Stylactis minoi
and the Scorpaenoid fish Minous inermis, which was first reported by me in the
Annals and Magazine of Natural History for September 1892, and has since
been abundantly verified.

      Many of the fishes of the Scorpaenoid family either live on the sea-
bottom or haunt the crevices of reefs, and are, by reason of their mottled
colouration and by a profusion of wavy cutaneous filaments, difficult to detect
among shingle and rocks encrusted by seaweed and zoophytes.

      Minous inermis has few cutaneous filaments, but is always more or less
invested with Hydroid polyps of one particular species. We have now taken the
fish at four widely distant localities and in considerable quantity, and every indi-
vidual has these polyps growing on its skin, especially in the neighbourhood of
the throat. There can be no doubt that the polyps are a useful disguise to the
fish, and that, on the other hand, their own attachment to a locomotive animal