As commensals there may be mentioned _Nereis fucata_, which lives in the upper coil of whelk-shells which are inhabited by a hermit crab. The same shell usually bears a particular sea-anemone, so that there are three animals living together in or upon the cast-off house of a fourth. _Siphonostoma_ is found in the "nests" made by the mollusc _Lima_. A _Eunice_ is constantly associated with the coral _Lophohelia prolifera_, amongst the branches of which the worm twines its tube; whilst another Polychaete inhabits a tube formed by the interweaving of the fine branches of the coral _Antipathes filix_,[358] found in the West Indian seas. A species of _Polydora_ forms its tube in _Heliopora_. The Polynoids present many instances of commensalism, a few of which may be here mentioned. _P. johnstoni_ Marenz. is only found in the tubes of _Terebella nebulosa_; other species occur in the tubes of other Terebellids. _P. marphysae_ lives in tubes of the Eunicid _Marphysa sanguinea_. Two species live in the tubes of _Chaetopterus_. _P. extenuata_ has been found in tubes of _Serpula vermicularis_, while _P. arenicolae_ occurs on {299}the body of the common lug-worm, with the colouring of which it closely harmonises. WORMS AS HOSTS.—The Polychaeta serve not only as food for fishes, Crustacea, and other predatory animals of larger size, but are also liable to be the hosts of parasites[359] such as Gregarines, and even, as we have seen, of other members of their own group. Sundry ectoparasitic Copepoda have been found attached to worms between the parapodia or to the sides of the feet, and an unnamed Copepod occurs attached, sometimes in considerable numbers, to the sides of _Nereis cultrifera_. The Polychaeta also act as protectors to other animals, for on the under surface of elytra of sundry Polynoids may very frequently be found specimens of _Loxosoma_, which may also be attached to gills of Eunicids; whilst below those of _Aphrodite echidna_ and _Hermadion pellucidum_, _Pedicellina belgica_ occurs. Under the felt of _A. aculeata_ the Sabellid _Branchiomma vigilans_ forms its tube, and Vorticellids may be found on chaetae, gills, or other parts of the body of sundry worms. DISTRIBUTION.—Very little can be said in a brief way of the _geographical distribution_ of these worms, for many of the genera are cosmopolitan, although only a few species occur in all the great oceans, _e.g._ _Polynoë imbricata_, _Hyalinoecia tubicola_, _Nerine_ (_Scolecolepis_) _cirrata_, and _Terebellides stroemi_.