A DEEP-SEA HERMIT-CRAB, Parapagurus pilosimanus, SHELTERED BY A COLONY OF Epizoanthus. FROM DEEP WATER OFF THE WEST OF IRELAND (SLIGHTLY REDUCED)

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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 triangular glandular patch." In a recent discussion of the whole question of phosphorescence in marine organisms, Dr. Doflein concludes that the part it plays in the life of the animal probably differs in the different cases. In some it may serve to attract prey, as moths are attracted to a candle; in others it may help individuals of the same species to keep together in a swarm or to find their mates, the varying arrangement of the photophores producing characteristic light-patterns that serve as "recognition marks" like the colour-patterns of animals that live in the light of day. The clouds of luminous secretion thrown out by Heterocarpus and other Prawns, and by certain Mysidacea and Ostracods, may serve to baffle pursuers, like the cloud of ink thrown out by a Cuttlefish, and in some cases the more complex organs may illuminate objects within the range of vision. That this does not exhaust the possibilities of speculation on the subject, however, is shown by the case of certain deep-sea Prawns which have been recently discovered to possess photophores placed so as to illuminate the interior of the gill cavities. What function they can discharge in this position seems beyond conjecture.

The colours of deep-sea Crustacea are very curious. Few of them have the blanched appearance common, for instance, in animals that live in the darkness of caves; on the contrary, their colours are often very vivid, but they are nearly always uniform, without spots or markings, and in a large proportion of cases are in some shade of red or orange. This red colour seems to be associated, in some way that we do not understand, with the darkness of their habitat. The general absence of markings is very striking. Dr. Alcock remarks that in deep-sea Crustacea we never see "those freaks of colour, or those labyrinthine mottlings and dapplings, that excite our curiosity when handling the Crabs and Shrimps of the reefs." Possibly the explanation of this may be that in these dwellers in darkness colour is merely, as it were, an accident, a by-product of physiological processes directed to other ends, not a character of protective or warning value, as in animals that hunt and are hunted in the light of day. It is a curious fact, which may have some bearing on this problem, that in many cases, while the adults are coloured in some shade of red, the eggs carried by the female are bright blue or green.

Some of the peculiarities of structure observed in deep-sea Crustacea seem to be correlated with the difficulties of resting or moving about with security on the soft ooze of the sea-floor. Among the Crabs we find a preponderance of long-legged species, not only among the true Spider Crabs (Oxyrhyncha), but also in other groups (Dromiacea like Latreillia, figured on [Plate XIX]., and Oxystomata), the members of which assume the same spider-like form. In some cases the legs are fringed with long stiff hairs, which may help to prevent the animal from sinking in the ooze, and the spines on the body and legs of many species may have the same effect. Among the deep-sea Prawns, the species of the family Nematocarcinidæ ([Plate XVII].) have extremely long and slender legs, which we may assume to be used like stilts for walking over the soft ooze.

PLATE XVII

A DEEP-SEA PRAWN, Nematocarcinus undulatipes. (SLIGHTLY REDUCED)

(From Brit. Mus. Guide)