Many deep-sea animals are known to be phosphorescent, and it seems probable that the large-eyed species may profit by the light emitted by the glow-worms and fireflies of the abyss. Thus, Alcock points out that the deep-sea Hermit Crab _Parapagurus pilosimanus_ (Plate XVI.), which lives in partnership with a colony of Sea-anemones which it carries about with it, has large eyes, although it descends to depths of at least 2,000 fathoms; and he suggests that the Crab may be able to see its way by the light emitted by the zoophytes. [Illustration: _PLATE XVI_ DEEP-SEA HERMIT-CRAB, _Parapagurus pilosimanus_, SHELTERED BY A COLONY OF _Epizoanthus_. FROM DEEP WATER OFF THE WEST OF IRELAND (SLIGHTLY REDUCED)] Some of the Crustacea, however, are themselves luminous. Thus, Alcock records how specimens of a deep-sea Prawn, _Heterocarpus alphonsi_, "poured out, apparently from the orifices of the 'green glands' at the base of the antennæ, copious clouds of a ghostly blue light of sufficient intensity to illuminate a bucket of sea-water so that all its contents were visible in the clearest detail." Certain other Prawns are known to possess special light-producing organs on various parts of the body and limbs. It is in the Euphausiacea, however, that these organs have been most fully examined, and although the members of this group (see Fig. 24, p. 56) are by no means all deep-sea animals, some of them occurring at the surface of the sea, the structure of their luminous organs, or "photophores," may appropriately be described here. They are situated on the under-surface of the abdomen, in the basal segments of some of the thoracic legs, and on the upper surface of the eye-stalks. Each consists of a globular capsule covered by a layer of pigment, except on the outer side, where there is a transparent biconvex lens. In the centre of the capsule is a peculiar "striated body" which seems to be the actual seat of luminescence, and behind it is a concave reflector composed of concentric lamellæ, and having a silvery lustre. Before their luminosity was observed, these organs were described as "accessory eyes," but there can be little doubt that they serve rather as searchlights, although, from the positions that some of them occupy on the body, it is not easy to see how they can illuminate objects within range of the eyes. That the function of phosphorescent organs is not always that of giving light for their possessor to see by is shown by the fact that many luminous animals are blind. It is important to notice, however, that these blind animals never have complex "photophores" like those just described, but only exhibit a diffuse luminosity or give off luminous secretions; as an example among Crustacea, the blind Eryonidea (see Fig. 46, p. 133) may be mentioned, one species of which was observed by Alcock to be "luminous at two points between the last pair of thoracic legs, where there is a