_Entocolax_ has been dredged from 180 fath. in Behring’s Straits, attached by its head to certain anterior muscles of a _Myriotrochus_.[169] A curious case of parasitism is described by Voeltzkow[170] as occurring on a _Synapta_ found between tide-marks on the I. of Zanzibar. In the oesophagus of the _Synapta_ was found a small bivalve (_Entovalva_), the animal of which was very large for its shell, and almost entirely enveloped the valves by its mantle. As many as five specimens occurred on a single _Synapta_. In the gut of the same Holothurian lived a small univalve, not creeping freely, but fixed to a portion of the stomach wall by a very long proboscis which pierced through it into the body cavity. This proboscis was nearly three times as long as the animal, and the forward portion of it was set with sharp thorns, no doubt in order to enable it to retain its hold and resist evacuation. Various species of _Eulima_ have been noticed in every part of the world, from Norway to the Philippines, both inside and outside Holothurians.[171] _Stilifer_ also occurs on this section of Echinoderms.[172] On _Annelida_.--_Cochliolepis parasiticus_ has been noticed under the scales of _Acoetes lupina_ (a kind of ‘sea-mouse’) in Charleston Harbour.[173] On _Crustacea_.--A mussel, ⅜ in. long, has been found[174] living under the carapace of the common shore-crab (_Carcinus maenas_), and one case has been noticed[175] where two mussels, one of several months’ growth, the other smaller, well secured by their byssi, were found under the abdomen of the same species, in such a position as to force the appendages apart and askew. These, however, are not so much cases of parasitism as of involuntary habitat, the mussel no doubt having become involved in the branchiae and the abdomen of the crab in the larval form. On _Mollusca_.--A species of _Odostomia_ (_pallida_ Mont.) is found on our own coasts on the ‘ears’ of _Pecten maximus_, and also[176] on the operculum of _Turritella communis_. Another species (_O. rissoides_) frequently occurs in hiding under beds of mussels, but it is not clear whether the habitat is due to parasitism, or simply to the fact that the mass of mussels, knitted together and to the rock by the byssi, affords the _Odostomia_ a safe lurking-place. At Panama the present writer found _Crepidula_ (2 sp.) plentiful on the opercula of the great _Strombus galea_ and of _Cerithium irroratum_. In each case the parasite exactly fitted the size of the operculum, and had assumed its colour, dark brown or chestnut. _Amalthea_ is very commonly found on _Conus_, _Turbo_, and other large shells from the South Pacific, but this is probably not a case of parasitism, but simply of convenience of habitat, just as young oysters are frequently seen on the carapace and even on the legs of large crabs. [Illustration: FIG. 30.--_Crepidula onyx_ Sowb., parasitic on the operculum of _Strombus galeatus_ Swains., Panama.]