ICHTHYOLOGY [DISXPaBUTION. apparently absent. This peculiarity has been observed in the Trackypteridce, Plagyodus, Chiasmodas, Melaiiocetus, Saccopharynx. But we cannot assume that it actually obtains whilst those fishes exist under their natural con ditions. Some of them are most rapacious creatures, which must be able to execute rapid and powerful movements to catch and overpower their prey ; and for that object their muscular system, thin as its layers may be, must be as lirm, and the chain of the segments of their vertebral column as firmly linked together as in surface fishes. It is evident, therefore, that the change which the body of those fishes has undergone on their withdrawal from the pressure under which they live is a much aggravated form of the affection that is experienced by persons reaching great altitudes in their ascent of a mountain or in a balloon. In every living organism with an intestinal tract there are accumulations of free gases ; and, moreover, the blood and other fluids, which penetrate every part of the body, contain gases in solution. Under greatly diminished pressure these gases expand, so that, if the withdrawal from a depth is not an extremely slow and gradual process, the various tissues must be distended, loosened, ruptured ; and what is a vigorous fish at a depth of 500 fathoms or more appears at the surface as a loosely-jointed body which, if the skin is not of sufficient toughness, can only be kept together with difficulty. At great depths a fibrous osseous structure and a thin layer of muscles suffice to obtain the same results for which, at the surface, thickness of muscle and firm osseous or cartilaginous tissue are necessary. The muciferous system of many deep-sea fishes is de veloped in an extraordinary degree. We find in fishes which are comparatively little removed from the surface (that is, to depths of from 100 to 200 fathoms) the lateral line much wider than in their congeners or nearest allies which live on the surface, as in Trackichthys, Hoplostcthus, many Scorpcenidcc, But in fishes inhabiting depths of 1000 fathoms and more the whole muciferous system is dilated ; it is especially the surface of the skull which is occupied by large cavities (Macrundce, deep-sea OpJddiidoe), and the whole body seems to be covered with a layer of mucus. These cavities collapse and shrink in specimens which have been preserved in spirit for some time, but a brief re- immersion in water generally suffices to show the immense quantity of mucus secreted by them. The physiological use of this secretion is unknown ; it has been observed to have phosphorescent properties in perfectly fresh specimens. The colours of deep-sea fishes are extremely simple, their bodies being either black or silvery ; in a few only are some filaments or the fin-rays of a bright scarlet colour. Among the black forms albinoes are not rare. The organ of sight is the first to be affected by a sojourn in deep water. Even in fishes which habitually live at a depth of only 80 fathoms, we find the eye of a propor tionally larger size than in their representatives at the surface. In such fishes the eyes increase in size with the depth inhabited by them, down to the depth of 200 fathoms, the large organs being necessary to collect as many rays of light as possible. Beyond that depth small-eyed as well as large-eyed fishes occur, the former having their want of vision compensated by tentacular organs of touch, whilst the latter have no such accessory organs, and evidently see only by the aid of phosphorescence. In the greatest depths blind fishes occur, with rudimentary eyes and without special organs of touch. Many fishes of the deep sea are provided with more or less numerous, round, shining, mother-of-pearl-coloured b-dies, imbedded in the skin. These so-called phosphores cent or luminous organs are either larger bodies of an oval or irregularly elliptical shape placed on the head, in the vicinity of the eye, or smaller round globular bodies arranged symmetrically in series along the side of the body and tail, especially near the abdominal profile, less frequently along the back. The former kind of organs possess in the interior a lenticular body, like the lens of an eye, and are considered by some naturalists true organs of vision (accessory eyes), the function of the latter, which have a glandular structure, being left unexplained by them. There is no doubt that the functions of these organs have some relation to the peculiar conditions of light under which the fishes provided with them live, these fishes being either deep-sea forms or nocturnal pelagic kinds. And it is highly probable that all produce and emit phosphorescent light, enabling the fishes to see in the darkness of the night or of the depths of the sea. Whenever we find in a fish long delicate filaments de veloped in connexion with the fins or the extremity of the tail, we may conclude that it is an inhabitant of still water and of quiet habits. Many deep-sea fishes (Trachypteridce, Macruridoe, Ophidiidce, Bathyptetrois) are provided with such filamentous prolongations, the development of which is perfectly in accordance with their sojourn in the absolutely quiet waters of abyssal depths. Some of the raptatorial deep-sea fishes have a stomach so distensible and capacious that it can receive a fish of twice or thrice the bulk of the destroyer (Melanocetus, Chiasmodus, iSaccopharynx). Deglutition is performed in them, not by means of the muscles of the pharynx, as in other fishes, but by the independent and alternate action of the jaws, as in snakes. These fishes cannot be said to swallow their food ; they rather draw themselves over their victim, alter the fashion of an Actinia. Before the voyage of H.M.S. " Challenger," scarcely thirty deep-sea fishes were known. This number is now much increased, six times as nriny new species and genera having been discovered. Modifications of certain organs, perfectly novel, and of the greatest inteiest, were found ; but, singularly, no new types of families were discovered, nothing but what might have been expected from our previous knowledge of this group of fishes. The fish fauna of the deep sea is chiefly composed of forms or modifications of forms which we find represented at the surface in the cold and temperate zones, or which belong to the class of nocturnal pelagic fishes. The Chon- dropterygians are few in number, not descending to a greater depth than 600 fathoms. The Acanthopterygians, which form the majority of the coast and surface faunas, are also scantily represented; genera identical with surface types are confined to the same inconsiderable depth as the Chondropterygians, whilst those Acanthopterygians which are so much specialized for a life in the deep sea as to deserve generic separation range from 200 to 2400 fathoms. Three distinct families of Acanthopterygians belong to the deep-sea fauna, viz., Trachypteridce, Lophotidce, and Nota- canthidce ; they consist of three, one, and two genera respectively. Gadidce, Ophidiida 1 , and Macruridce are very numerous, ranging through all depths ; they constitute about one- fourth of the whole deep-sea fauna. Of Physostomi, the families of Sternoptychidas, Scopelidw, ( Stomiatid<x, Salmonida>, Bathythrissidce, Alrpoceph didce, Ifalosauridce, and Murcenidce are represented. Of these the Scopeloids are the most numerous, constituting nearly another fourth of the fauna. Salmonidcn are only repre sented by three small genera. Bathytkrisxidce include one species only, which is probably confined in its vertical as well as its horizontal range ; it occurs at a depth of about 350 fathoms in the sea of Japan. The Alepocephalidce and I/alosauridcp, known before the " Challenger " expedition
from isolated examples only, prove to be true, widely-
