Chapter XXI: The Fishes of the Deep Sea (1)

The knowledge of the existence of deep-sea fishes is one of the recent discoveries of ichthyology. It is only about twenty years ago that, from the evidence afforded by the anatomical structure of a few singular fishes obtained in the North Atlantic, an opinion was expressed that these fishes inhabited great depths of the ocean, and that their organisation was specially adapted for living under the physical abyssal conditions. These fishes agreed in the character of their connective tissue, which was so extremely weak as to yield to, and to break under, the slightest pressure, so that the greatest difficulty is experienced to preserve their body in its continuity. Another singular circumstance was, that some of the specimens were picked up floating on the surface of the water, having met their deaths whilst engaged in swallowing or digesting another fish not much inferior or even superior in size to themselves.

The first peculiarity was accounted for by the fact that, if those fishes really inhabited the great depths supposed, their removal from the enormous pressure under which they lived would be accompanied by such an expansion of gases within their tissues as to rupture them, and to cause a separation of the parts which had been held together by the pressure. The second circumstance was explained thus:--A raptatorial fish organised to live at a depth of between 500 and 800 fathoms seizes another usually inhabiting a depth of between 300 and 500 fathoms. In its struggles to escape, the fish seized, nearly as large or strong as the attacking fish, carries the latter out of its depth into a higher stratum, where the diminished pressure causes such an expansion of gases as to make the destroyer with its victim rise with increasing rapidity towards the surface, which they reach dead or in a dying condition. Specimens in this condition are not rarely picked up; and as, of course, comparatively few can by accident fall into the hands of naturalists, occurrences of the kind related must happen very often.

Thus, the existence of fishes peculiarly adapted for the deep sea has been a fact maintained and admitted for some time in Ichthyology; and as the same genera and species were found at very distant parts of the ocean, it was further stated that those Deep-sea fishes were not limited in their range, and that, consequently, the physical conditions of the depths of the ocean must be the same or nearly the same over the whole globe. That Deep-sea fishes were not of a peculiar order, but chiefly modified forms of surface types, was another conclusion arrived at from the sporadic evidence collected during the period which preceded systematic deep-sea dredging.

However, nothing was positively known as to the exact depths inhabited by those fishes until observations were made during the voyage of H.M.S. “Challenger.” The results obtained by this expedition afforded a surer and more extended basis for our knowledge of Deep-sea fishes.

The physical conditions of the deep sea, which must affect the organisation and distribution of fishes, are the following:--

1. Absence of sunlight. Probably the rays of the sun do not penetrate to, and certainly do not extend beyond, a depth of 200 fathoms, therefore we may consider this to be the depth where the Deep-sea fauna commences. Absence of light is, of necessity, accompanied by modifications of the organs of vision and by simplification of colours.

2. The absence of sunlight is in some measure compensated for by the presence of phosphorescent light, produced by many marine animals, and also by numerous Deep-sea fishes.

3. Depression and equality of the temperature. At a depth of 500 fathoms the temperature of the water is already as low as 40° Fahr., and perfectly independent of the temperature of the surface-water; and from the greatest depths upwards to about 1000 fathoms the temperature is uniformly but a few degrees above freezing-point. Temperature, therefore, ceases to offer an obstacle to the unlimited dispersal of Deep-sea fishes.

4. The increased pressure by the water. The pressure of the atmosphere on the level of the sea amounts to fifteen pounds per square inch of the surface of the body of an animal; but the pressure amounts to a ton weight for every 1000 fathoms of depth.

5. With the sunlight, vegetable life ceases in the depths of the sea. All Deep-sea fishes are therefore carnivorous; the most voracious feeding frequently on their own offspring, and the toothless kinds being nourished by the animalcules which live on the bottom, or which, “like a constant rain,” settle down from the upper strata towards the bottom of the sea.

6. The perfect quiet of the water at great depths. The agitation of the water, caused by the disturbances of the air, does not extend beyond the depth of a few fathoms; below this surface-stratum there is no other movement except the quiet flow of ocean-currents, and near the bottom of the deep sea the water is probably in a state of almost entire quiescence.

The effect upon fishes of the physical conditions described is clearly testified by the modification of one or more parts of their organisation, so that every Deep-sea fish may be recognised as such, without the accompanying positive evidence that it has been caught at a great depth; and _vice versa_, fishes reputed to have been obtained at a great depth, and not having any of the characteristics of the dwellers of the deep sea, must be regarded as surface-fishes.

The most striking characteristic, found in many Deep-sea fishes, is in relation to the tremendous pressure under which they live. Their osseous and muscular systems are, as compared with the same parts of surface-fishes, very feebly developed. The bones have a fibrous, fissured, and cavernous texture; are light, with scarcely any calcareous matter, so that the point of a needle will readily penetrate them without breaking. The bones, especially the vertebræ, appear to be most loosely connected with one another; and it requires the most careful handling to prevent the breaking of the connective ligaments. The muscles, especially the great lateral muscles of the trunk and tail, are thin, the fascicles being readily separated from one another or torn, the connective tissue being extremely loose, feeble, or apparently absent. This peculiarity has been observed in the _Trachypteridæ_, _Plagyodus_, _Chiasmodus_, _Melanocetus_, _Saccopharynx_. But we cannot assume that it actually obtains whilst those fishes exist under their natural conditions. 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 firm, and the chain of the segments of their vertebral column as firmly linked together as in surface-fishes. Therefore, it is evident 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 permeate 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 or more fathoms, 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 suffices 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 developed in an extraordinary degree. We find already in fishes which are comparatively little removed from the surface (that is to depths of 100–200 fathoms), the lateral line much wider than in their congeners or nearest allies which live on the surface, as in _Trachichthys_, _Hoplostethus_, many _Scorpænidæ_. But in fishes inhabiting depths of 1000 and more fathoms, the whole muciferous system is dilated; it is especially the surface of the skull which is occupied by large cavities (_Macruridæ_, deep-sea _Ophidiidæ_), 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 re-immersion in water for a short time 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 scarce.

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 proportionally 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 eyes being necessary to collect as many rays of light as possible. Beyond that depth small-eyed fishes as well as large-eyed occur, the former having their want of vision compensated for 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 bodies, imbedded in the skin. These so-called phosphorescent 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 have not yet been anatomically examined. The number of pairs of the latter is in direct relation to that of the segments of the vertebral column, the muscular system, etc. (meta*-meres); and two kinds may be distinguished differing from each other in their anatomical structure. The organs of one kind consist of an anterior, biconvex, lens-like body, which is transparent during life, simple or composed of rods (_Chauliodus_); and of a posterior chamber which is filled with a transparent fluid, and coated with a dark membrane composed of hexagonal cells, or of rods arranged as in a retina. This structure is found in _Astronesthes_, _Stomias_, _Chauliodus_, etc. In the other kind the organ shows throughout a simply glandular structure, but apparently without an efferent duct (_Gonostoma_, _Scopelus_, _Maurolicus_, _Argyropelecus_). Branches of the spinal nerves run to each organ, and are distributed over the retina-like membrane or the glandular follicles. The former kind of organs are considered by some naturalists true organs of vision (accessory eyes), the function of the latter being left unexplained by them.

Although, thus, these organs morphologically differ from each other, there is no doubt that the functions of all 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. There are three possible hypotheses as to the function of these organs:--

1. All the different kinds of organs are sensory, or, in other words, accessory eyes.

2. Only the organs with a lenticular body are sensory, and those with a glandular structure produce and emit phosphorescent light.

3. All are producers of light.

There are very serious objections to adopting the first view. _Scopelus_ and _Argyropelecus_ possess not only perfectly developed, but even large eyes, specially adapted for a nocturnal life; and therefore accessory organs of vision must appear to be quite superfluous to them. On the other hand, in Deep-sea fishes without external eyes, which would seem to especially require these metameric organs of sense, they are invariably absent. And, finally, it is quite inconceivable that the glandular structures should have the faculty of conveying impressions of light to the nervous centre. The second supposition seems therefore to be nearer the truth; and is supported by the fact that the glandular organs of Scopeli have actually been observed to gleam with phosphorescent light, and by the obvious morphological similarity of the organs with a lenticular body and retina-like membrane to an organ of vision. We are, moreover, justified, from an _à priori_ consideration, in supposing that in depths to which no sunlight descends, and which are illuminated by phosphorescent light only, peculiar organs of vision would have been developed. On the other hand, this supposition is opposed by the fact that many fishes which dwell in those abyssal depths are provided with large ordinary eyes (as the _Trachypteri_, the majority of _Macruridæ_), and, therefore, that the ordinary organ of vision is quite sufficient for seeing by phosphorescent light. Thus, whilst we must admit that those compound organs may prove to be organs of sense, we maintain at the same time that their morphological nature is not opposed to the belief that they too, like the glandular organs, are producers of light. It may be produced at the bottom of the posterior chamber, and emitted through the lenticular body in particular directions, with the same effect as light is sent through the convex glass of a “bull’s eye.” This hypothesis seems to be less bold than the other, which would require us to believe that vertebrate animals, with a nervous centre specialised for the reception of the impressions of the higher senses, should receive them through the spinal chord.

[See _Ussow_, “Ueber den Bau der sogenannten
augenaehnlichen Flecken einiger Knochenfische.” St. Petersburg,
Bullet. 1879.]

Whenever we find in a fish long delicate filaments, developed in connection 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 (_Trachypteridæ_, _Macruridæ_, _Ophidiidæ_, _Bathypterois_) 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_, _Saccopharynx_). 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, but rather draw themselves over their victim, in 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 by the discovery of many new species and genera; 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. Modifications of certain organs, perfectly novel, and of the greatest interest, were found, as we shall see in the “Systematic Part;” but the most important results of this voyage are that the general character of the abyssal fish-fauna, the abundance of fishes, and the exact depths to which fishes may descend, have been ascertained.

However, the statements of the depths at which the fishes collected by the “Challenger” were taken cannot be received without some critical examination of each individual species. No precaution was taken to keep the mouth of the dredge closed during its descent or ascent, and therefore it is quite within the limits of probability that sometimes fishes were accidentally enclosed within the dredge, whilst it was traversing the surface strata. And this has happened more than once; for it is quite certain that common surface fishes like _Sternoptyx_ and _Astronestles_, never ranged to a depth of 2500 fathoms. On the other hand, the majority of the fishes obtained offer sufficient evidence from their own organisation that they live on the bottom, and are unable to support themselves in the water at a certain distance from the bottom or surface; and, consequently, that they actually were obtained at the depth to which the dredge descended.

As far as the observations go at present, no distinct bathymetrical regions, which would be characterised by peculiar forms, can be defined. The depths from 200 to 600 fathoms are inhabited by numerous forms, still strongly reminding us of surface types. To this fauna belong the few Chondropterygians of the deep sea, a Sebastes and Setarches, a Beryx and Polymixia, a Cottus, etc.; but they are associated with many others which descend to the greatest depths. And before anything like a division into bathymetrical zones can be attempted, the observations of the “Challenger” expedition must be confirmed and supplemented by other series of similar systematic observations. One of the most startling conclusions at which we would have to arrive from the “Challenger” observations is, that some of the species of Deep-sea fishes would range from a depth of some 300 fathoms down to one of 2000 fathoms; or, in other words, that a fish which has once attained in its organisation to that modification by which it is enabled to exist under the pressure of half a ton, can easily accommodate itself to one of two tons or more,--a conclusion which is not in accordance with anatomical facts, and which must be confirmed by other observations before we can adopt it. But if the vertical range of Deep-sea fishes is actually as it appears from the “Challenger” lists, then there is no more distinct vertical than horizontal distribution of Deep-sea fishes.

The greatest depth reached hitherto by a dredge in which fishes were enclosed is 2900 fathoms. But the specimens thus obtained belong to a species (_Gonostoma microdon_), which seems to be extremely abundant in upper strata of the Atlantic and Pacific, and were therefore most likely caught by the dredge in its ascent. The next greatest depth, viz., 2750 fathoms, must be accepted as one at which fishes undoubtedly do live; the fish obtained from this depth of the Atlantic, _Bathyophis ferox_, showing by its whole habit that it is a form living on the bottom of the ocean.

The fish-fauna of the deep sea is composed chiefly of forms or modifications of forms which we find represented at the surface in the cold and temperate zones, or which appear as nocturnal pelagic forms. The Chondropterygians are few in number, not descending to a depth of more 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 depths as the Chondropterygians, whilst those Acanthopterygians which are so much specialised for the 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. _Trachypteridæ_, _Lophotidæ_, and _Notacanthidæ_; they respectively consist of three, one, or two genera only.

_Gadidæ_, _Ophidiidæ_, and _Macruridæ_ are very numerous, ranging through all depths; they constitute about one-fourth of the whole deep-sea fauna.

Of _Physostomi_, the families of _Sternoptychidæ_, _Scopelidæ_, _Stomiatidæ_, _Salmonidæ_, _Bathythrissidæ_, _Alepocephalidæ_, _Halosauridæ_, and _Murænidæ_ are represented. Of these the _Scopeloids_ are the most numerous, constituting nearly another fourth of the fauna. _Salmonidæ_ are scarce, with three small genera only. _Bathythrissidæ_ include one species only, which is probably confined in its vertical as well as horizontal range; it occurs at a depth of about 350 fathoms in the sea of Japan. The _Alepocephalidæ_ and _Halosauridæ_, known before the “Challenger” expedition from isolated examples only, prove to be true, widely-spread, deep-sea types. Eels are well represented, and seem to descend to the greatest depths.

_Myxine_ has been obtained from a depth of 345 fathoms.

It will be useful to append a complete list of Deep-sea fishes, with the depths as ascertained by the dredgings of the “Challenger:”--

_List of Deep-sea Fishes._

Fathoms.

CHONDROPTERYGIANS--
Raja 565
Scyllium 400
Centroscyllium 245
Centrophorus 345–500

ACANTHOPTERYGIANS--
Pomatomus (? down to) 200
Sebastes 275
Setarches 215
Beryx 345
Melamphaes (? beyond) 200
Polymixia 345
Nealotus
Nesiarchus
Aphanopus
Euoxymetopon
Lepidopus 345
Gempylus
Anomalops
? Antigonia
Diretmus
Cottus 565
Bathydraco 1260
Oneirodes
Melanocetus johnsonii 1850
„ bispinosus 360
Himantolophus
Chaunax 215
Ceratias 2400
Halieutichthys
Dibranchus 360
Trachypteridæ
Lophotes
Notacanthus rissoanus 1875
„ bonapartii 400

ANACANTHINI--

Melanonus 1975
Halargyreus
Lotella marginata 120–345
Physiculus 345
Uraleptus
Læmonema
Haloporphyrus australis 55–70
„ lepidion 345–600
„ rostratus 600–1375
Chiasmodus niger 1500
Sirembo grandis 1875
„ macrops 375
„ messieri 345
„ ocellatus 350
„ brachysoma 350
Acanthonus armatus 1075
Typhlonus nasus 2440
Aphyonus gelatinosus 1400
Rhinonus ater 2150
Bathynectes laticeps 2500
„ compressus 1075–2500
„ gracilis 1400
Pteridium
Macrurus (12 species) 120–700
Coryphænoides norvegicus
„ serratus
„ nasutus 345–565
„ villosus 345
„ rudis 500–650
„ æqualis 600
„ crassiceps 520–650
„ microlepis 215
„ murrayi 1100
„ serrulatus 700
„ filicauda 1800–2650
„ variabilis 1375–2425
„ affinis 1900
„ carinatus 500
„ longifilis 565
„ altipinnis 565–1875
„ asper 500–1875
„ leptolepis 350–2050
„ sclerorhynchus 1090
„ denticulatus 275–520
Malacocephalus 350
Bathygadus cottoides 520–700
„ multifilis 500

STERNOPTYCHIDÆ--
Argyropelecus 1127 [?]
Sternoptyx 0–2500 [?]
Polyipnus 255
Gonostoma denudatum
„ microdon 500–2900 [?]
„ elongatum 360–800
„ gracile 345–2425
Chauliodus 565–2560

SCOPELIDÆ--
Bathysaurus ferox 1100
„ mollis 1875–2385
Bathypterois longifilis 520–630
„ longipes 2650
„ quadrifilis 500–770
„ longicauda 2550
Chlorophthalmus agassizii 215
„ nigripinnis 120
„ gracilis 1100–1425
Scopelus engraulis 255
„ antarcticus 1950
„ mizolepis 800
„ dumerilii 215
„ macrolepidotus 520–630
„ crassiceps 675–1550
„ macrostoma 2350–2425
„ microps 1375
Odontostomus hyalinus
Odontostomus humeralis 500
Nannobrachium nigrum 500
Ipnops murrayi 1600–2150
Paralepis
Sudis
Plagyodus

STOMIATIDÆ--
Astronesthes niger 2500 [?]
Stomias boa 450–1800
„ barbatus
„ ferox
Echiostoma barbatum
„ micripnus 2150
„ microdon 2440
Malacosteus niger
„ indicus 500
Bathyophis ferox 2750

SALMONIDÆ--
Argentina
Microstoma
Bathylagus antarcticus 1950
„ atlanticus 2040

BATHYTHRISSIDÆ--
Bathythrissa dorsalis 345

ALEPOCEPHALIDÆ--
Alepocephalus rostratus
„ niger 1400
Platytroctes apus 1500
Bathytroctes microlepis 1090
„ rostratus 675
„ macrolepis 2150
Xenodermichthys 345

HALOSAURIDÆ--
Halosaurus owenii
„ affinis 565
„ macrochir 1090–1375
„ mediorostris 700
„ rostratus 2750

MURÆNIDÆ--
Nemichthys scolopacea
„ infans 500–2500
Cyema atrum 1500–1800
Saccopharynx
Synaphobranchus pinatus 345–1200
„ bathybius 1875–2050
„ brevidorsalis 1075–1375
„ affinis 345
Nettastoma parviceps 345

CYCLOSTOMATA--
Myxine australis 345

SYSTEMATIC AND DESCRIPTIVE PART.

The Class of Fishes is divided into four sub-classes:--

I. PALÆICHTHYES.--Heart with a contractile conus arteriosus; intestine with a spiral valve; optic nerves non-decussating, or only partially decussating.

II. TELEOSTEI.--Heart with a non-contractile bulbus arteriosus; intestine without spiral valve; optic nerves decussating. Skeleton ossified, with completely separated vertebræ.

III. CYCLOSTOMATA.--Heart without bulbus arteriosus; intestine simple. Skeleton cartilaginous and notochordal. One nasal aperture only. No jaws; mouth surrounded by a circular lip.

IV. LEPTOCARDII.--Heart replaced by pulsating sinuses; intestine simple. Skeleton membrano-cartilaginous and notochordal. No skull; no brain.

FIRST SUB-CLASS: PALÆICHTHYES.

_Heart with a contractile conus arteriosus;[30] intestine with a spiral valve;[31] optic nerves non-decussating, or only partially decussating;[32] skeleton cartilaginous or osseous._

This sub-class comprises the Sharks and Rays, and the Ganoid fishes. Although based upon a singular concurrence of most important characters, its members exhibit as great a diversity of form, and as manifold modifications in the remainder of their organisation as the _Teleostei_. The _Palæichthyes_ stand to the _Teleostei_ in the same relation as the Marsupials to the Placentalia. Geologically, as a sub-class, they were the predecessors of Teleosteous fishes; and it is a remarkable fact that all those modifications which show an approach of the ichthyic type to the Batrachians are found in this sub-class. We divide it into two orders: _Chondropterygii_ and _Ganoidei_.

FIRST ORDER: CHONDROPTERYGII.

_Skeleton cartilaginous. Body with medial and paired fins, the hinder pair abdominal. Vertebral column generally heterocercal, the upper lobe of the caudal fin produced. Gills attached to the skin by the outer margin, with several intervening gill-openings: rarely one external gill-opening only. No gill-cover. No air-bladder. Two, three, or more series of valves in the conus arteriosus. Ova large and few in number,[33] impregnated and, in some species, developed within a uterine cavity. Embryo with deciduous external gills.[34] Males with intromittent organs attached to the ventral fins.[35]_

This order, for which, also, the name _Elasmobranchii_ has been proposed (by Bonaparte), comprises the Sharks and Rays and Chimæras, and is divided into two suborders: _Plagiostomata_ and _Holocephala_.

FIRST SUB-ORDER: PLAGIOSTOMATA.

_From five to seven gill-openings. Skull with a suspensorium and the palatal apparatus detached. Teeth numerous._

The Plagiostomes differ greatly among each other with regard to the general form of their body: in the Sharks or _Selachoidei_ the body is elongate, more or less cylindrical, gradually passing into the tail; their gill-openings are lateral. In the Rays, or _Batoidei_, the gill-openings are always placed on the abdominal aspect of the fish; the body is depressed, and the trunk, which is surrounded by the immensely developed pectoral fins, forms a broad flat disk, of which the tail appears as a thin and slender appendage. Spiracles are always present; the number of gill-openings is constantly five; no anal fin; dorsal fins, if present, situated on the tail. However, some of the Rays approach the Sharks in having the caudal portion less abruptly contracted behind the trunk.

Fossil Plagiostomes are very numerous in all formations. Some of the earliest determinable fish remains are believed to be, or are, derived from Plagiostomes. Those which can be referred to any of the following families will be mentioned subsequently: but there are others, especially fin-spines, which leave us in doubt to which group of Plagiostomes their owners had any affinity, thus _Onchus_ from the upper Silurian, continuing to carboniferous formations; _Dimeracanthus_, _Homocanthus_, from the Devonian; _Oracanthus_, _Gyracanthus_, _Tristychius_, _Astroptychius_, _Ptychacanthus_, _Sphenacanthus_, etc., from carboniferous formations; _Leptacanthus_, from the coal to the Oolite; _Cladacanthus_, _Cricacanthus_, _Gyropristis_, and _Lepracanthus_, from the coal measures; _Nemacanthus_, _Liacanthus_, from the Trias; _Astracanthus_, _Myriacanthus_, _Pristacanthus_, from the Jurassic group.

A. SELACHOIDEI: SHARKS.

The elongate cylindrical body, generally terminating in a more or less pointed snout, and passing into a powerful and flexible tail, blade-like at its extremity, gives to the Sharks a most extraordinary power of swimming, with regard to endurance as well as rapidity of motion. Many, especially the larger kinds, inhabit the open ocean, following ships for weeks, or pursuing shoals of fishes in their periodical migrations. Other large-sized sharks frequent such parts of the coast as offer them abundance of food; whilst the majority of the smaller kinds are shore fishes, rarely leaving the bottom, and sometimes congregating in immense numbers. The movements of sharks resemble in some measure those of snakes, their flexible body being bent in more than one curve when moving.

Sharks are most numerous in the seas between the Tropics, and become scarcer beyond, a few only reaching the Arctic circle; it is not known how far they advance southwards towards the Antarctic region. Some species enter fresh waters, and ascend large rivers, like the Tigris or Ganges, to a considerable distance. The pelagic as well as the shore species have a wide geographical range. Very few descend to a considerable depth, probably not exceeding 500 fathoms. There are about 140 different species known.

Sharks have no scales like those of other fishes; their integuments are covered with calcified papillæ which, under the microscope, show a structure similar to that of teeth. If the papillæ are small, pointed, and close set, the skin is called “shagreen;” rarely they are larger, appearing as bucklers or spines, of various sizes.

These fishes are exclusively carnivorous, and those armed with powerful cutting teeth are the most formidable tyrants of the ocean. They have been known to divide the body of a man in two at one bite, as if by the sweep of a sword. Some of the largest sharks, however, which are provided with very small teeth, are almost harmless, feeding on small fishes only or marine invertebrates. Others, particularly of the smaller kinds, commonly called “Dog-fishes,” have short or obtuse teeth, and feed on shells or any other animal substance. Sharks scent their food from a distance, being readily attracted by the smell of blood or decomposing bodies.

In China and Japan, and many other eastern countries, the smaller kinds of sharks are eaten. Sharks’ fins form in India and China a very important article of trade, the Chinese preparing from them gelatine, and using the better sorts for culinary purposes. The fins are obtained not exclusively from Sharks but also from Rays, and assorted in two kinds, viz. “white and black.” The white consist exclusively of the dorsal fins, which are on both sides of the same uniform light colour, and reputed to yield more gelatine than the other fins. The pectoral, ventral, and anal fins pass under the denomination of black fins; the caudal fin is not used. One of the principal places where shark fishery is practised as a profession is Kurrachee. Dr. Buist, writing in 1850 (“Proc. Zool. Soc.” 1850, p. 100), states that there are thirteen large boats, with crews of twelve men each, constantly employed in this pursuit; that the value of the fins sent to the market varies from 15,000 to 18,000 rupees; that one boat will sometimes capture at a draught as many as one hundred sharks of various sizes; and that the number total of sharks captured during the year amounts probably to not less than 40,000. Large quantities are imported from the African coast and the Arabian Gulf, and various ports on the coast of India. In the year 1845–46, 8770 cwt. of sharks’ fins were exported from Bombay to China.

FIRST FAMILY--CARCHARIIDÆ.

_Eye with a nictitating membrane. Mouth crescent-shaped, inferior. Anal fin present. Two dorsal fins, the first opposite to the space between pectoral and ventral fins, without spine in front._

CARCHARIAS.--Snout produced in the longitudinal axis of the
body; mouth armed with a series of large flat triangular teeth,
which have a smooth cutting or serrated edge. Spiracles absent.
A transverse pit on the back of the tail, at the root of the
caudal fin.

This genus comprises the true Sharks, common in the tropical, but less so in the temperate seas. Between thirty and forty different species have been distinguished, of which one of the most common is the “Blue Shark” (_Carcharias glaucus_). Individuals of from twelve to fifteen feet are of very common occurrence, but some of the species attain a much larger size, and a length of 25 and more feet. Fishes of this genus or of closely allied genera (_Corax_, _Hemipristis_) are not uncommon in the chalk and tertiary formations.

GALEOCERDO.--Teeth large, flat, triangular, oblique, serrated
on both edges, with a deep notch on the outer margin. Spiracles
small. A pit on the tail, above and below, at the root of the
caudal fin. Two notches on the under caudal border, one of them
at the end of the spine.

Three species, of which one (_G. arcticus_) is confined to the arctic and sub-arctic oceans. The others inhabit temperate and tropical seas, and all attain to a very large size.

GALEUS.--Snout produced in the longitudinal axis of the
body; teeth equal in both jaws, rather small, flat, triangular,
oblique, serrated and with a notch. Spiracles small. No pit at
the commencement of the caudal fin, which has a single notch on
its lower margin.

These are small sharks, commonly called “Tope.” The species found on the British coast is spread over nearly all the temperate and tropical seas, and is common in California and Tasmania. It lives on the bottom, and is very troublesome to fishermen by constantly taking away bait or driving away the fishes which they desire to catch.

ZYGÆNA.--The anterior part of the head is broad, flattened, and
produced into a lobe on each side, the extremity of which is
occupied by the eye. Caudal fin with a single notch at its lower
margin. A pit at the root of the caudal fin. Spiracles none.
Nostrils situated on the front edge of the head.

The “Hammerheads,” or Hammerheaded Sharks, have a dentition very similar to that of _Carcharias_, and although they do not attain to the same large size, they belong to the most formidable fishes of the ocean. The peculiar form of their head is quite unique among fishes; young examples have the lateral extension of the skull much less developed than adults. Five species are known, which are most abundant in the tropics. By far the most common is _Zygæna malleus_, which occurs in nearly all tropical and sub-tropical seas. Specimens of this species may be often seen ascending from the clear blue depths of the ocean like a great cloud. Cantor found in a female, nearly 11 feet long, thirty-seven embryons.--Hammerheads have lived from the cretaceous epoch.

MUSTELUS.--The second dorsal fin is not much smaller than the
first. No pit at the root of the caudal, which is without
distinct lower lobe. Snout produced in the longitudinal axis
of the body. Spiracles small, behind the eyes. Teeth small,
numerous, similar in both jaws, obtuse, or with very indistinct
cusps, arranged like pavement.

The “Hounds” are small Sharks, abundant on the coasts of all the temperate and tropical seas; two of the five species known occur on the coasts of Europe, viz. _M. lævis_ and _M. vulgaris_. Closely allied as these two species are, they yet show a most singular difference, viz. that a placenta is developed in the uterus for the attachment of the embryo in _M. lævis_ (the Γαλεὁς λεȋος of Aristotle, to whom this fact was already known); whilst the embryons of _M. vulgaris_ are developed without such placenta (see _J. Müller_, “Abhandl. Ak. Wiss.” Berl. 1840). The Hounds are bottom fish, which feed principally on shells, crustaceans, and decomposing animal substances.

* * * * *

Several other genera belong to the family Carchariidæ, but it will be sufficient to mention their names:--_Hemigaleus_, _Loxodon_, _Thalassorhinus_, _Triænodon_, _Leptocarcharias_, and _Triacis_.

SECOND FAMILY--LAMNIDÆ.

_Eye without nictitating membrane. Anal fin present. Two dorsal fins; the first opposite to the space between pectoral and ventral fins, without spine in front. Nostrils not confluent with the mouth which is inferior. Spiracles absent or minute._

All the fishes of this family attain to a very large size, and are pelagic. But little is known of their reproduction. The first appearance of this family is indicated by _Carcharopsis_, a genus from carboniferous formations, the teeth of which differ from those of _Carcharodon_ only by having a broad fold at the base. In the chalk and tertiary formations almost all the existing genera are represented; and, besides, _Oxytes_, _Sphenodus_, _Gomphodus_, and _Ancistrodon_, which are known from teeth only, have been considered generically distinct from the living Porbeagles.

LAMNA (OXYRHINA).--The second dorsal and anal are very small.
A pit at the root of the caudal, which has the lower lobe much
developed. Side of the tail with a prominent longitudinal keel.
Mouth wide. Teeth large, lanceolate, not serrated, sometimes
with additional basal cusps. On each side of the upper jaw, at
some distance from the symphysis, there is one or two teeth
conspicuously smaller than the others. Gill-openings very wide.
Spiracles minute.

Of the “Porbeagles,” three species have been described, of which the one occurring in the North Atlantic, and frequently straying to the British coasts (_L. cornubica_), is best known. It attains to a length of ten feet, and feeds chiefly on fishes; its lanceolate teeth are not adapted for cutting, but rather for seizing and holding its prey, which it appears to swallow whole. According to Pennant it is viviparous; only two embryoes were found in the female which came under his observation. Haast has found this species also off the coast of New Zealand.

CARCHARODON.--The second dorsal and anal are very small. Pit at
the root of the caudal, which has the lower lobe well developed.
Side of the tail with a prominent longitudinal keel. Mouth wide.
Teeth large, flat, erect, regularly triangular, serrated. On
each side of the upper jaw, at some distance from the symphysis,
there is one or two teeth conspicuously smaller than the others.
Gill-openings wide.

One species only is known (_C. rondeletii_), which is the most formidable of all Sharks. It is strictly pelagic; and appears to occur in all tropical and sub-tropical seas. It is known to attain to a length of 40 feet. The tooth figured here, of the natural size, is taken from a jaw 20 inches wide in its transverse diameter (inside measure), each half of the mandible measuring 22 inches.[36] The whole length of the fish was 36½ feet.

Carcharodon teeth are of very common occurrence in various tertiary strata, and have been referred to several species, affording ample evidence that this type was much more numerously represented in that geological epoch than in the recent fauna. Some individuals attained to an immense size, as we may judge from teeth found in the Crag, which are 4 inches wide at the base, and 5 inches long, measured along their lateral margin. The naturalists of the “Challenger” expedition have made the highly interesting discovery that teeth of similar size are of common occurrence in the ooze of the Pacific, between Polynesia and the west coast of America. As we have no record of living individuals of that bulk having been observed, the gigantic species to which these teeth belonged must have become extinct within a comparatively recent period. Nothing is known of the anatomy, habits, and reproduction of the surviving species, and no opportunity should be lost of obtaining information on this Shark.

ODONTASPIS.--The second dorsal and anal are not much smaller
than the first dorsal. No pit at the root of the caudal. Side of
the tail without keel. Mouth wide. Teeth large, awl-shaped, with
one or two small cusps at the base. Gill-openings of moderate
width.

Large Sharks from tropical and temperate seas; two species.

ALOPECIAS.--The second dorsal and anal very small. Caudal fin
of extraordinary length, with a pit at its root. No keel on the
side of the tail. Mouth and gill-openings of moderate width.
Teeth equal in both jaws, of moderate size, flat, triangular,
not serrated.

This genus consists of one species only, which is known by the name of “Fox-shark” or “Thresher.” It is the most common of the larger kinds of Sharks which occur on the British coasts; and seems to be equally common in other parts of the Atlantic and Mediterranean, as well as on the coasts of California and New Zealand. It attains to a length of fifteen feet, of which the tail takes more than one half; and is quite harmless to man. It follows the shoals of Herrings, Pilchards, and Sprats in their migrations, destroying incredible numbers. When feeding it uses the long tail in splashing the surface of the water, whilst it swims in gradually decreasing circles round a shoal of fishes, which are thus kept crowded together, falling an easy prey to their enemy. Statements that it has been seen to attack Whales and other large Cetaceans, rest upon erroneous observations.

SELACHE.--The second dorsal and anal very small. A pit at the
root of the caudal fin, which is provided with a lower lobe.
Side of the tail with a keel. Gill-openings extremely wide.
Teeth very small, numerous, conical, without serrature or
lateral cusps.

Also this genus consists of one species only, the “Basking Shark” (Pélerin of the French). It is the largest Shark of the North Atlantic, growing to a length of more than thirty feet. It is quite harmless if not attacked; its food consisting of small fishes, and other small marine animals swimming in shoals. On the west coast of Ireland it is chased for the sake of the oil which is extracted from the liver, one fish yielding from a ton to a ton and a-half. Its capture is not unattended with danger, as one blow from the enormously strong tail is sufficient to stave in the sides of a large boat. At certain seasons it is gregarious, and many specimens may be seen in calm weather lying together motionless, with the upper part of the back raised above the surface of the water; a habit from which this Shark has derived its name. The buccal and branchial cavities are of extraordinary width, and, in consequence of the flabby condition of those parts, the head presents a variable and singular appearance in specimens lying dead on the ground. This peculiarity, as well as the circumstance that young specimens have a much longer and more pointed snout than adult ones, has led to the erroneous opinion that several different genera and species of Basking Shark occur in the European seas. The branchial arches of _Selache_ are provided with a very broad fringe of long (five to six inches) and thin gill-rakers, possessing the same microscopical structure as the teeth and dermal productions of Sharks. Similar gill-rakers have been found in a fossil state in the Crag of Anvers in Belgium, proving the existence of this Selachian type in the tertiary epoch. Nothing is known of the reproduction of this fish. The latest contributions to its history are by _Steenstrup_ in “Overs. Dansk. Vidensk. Selsk., Forhandl.” 1873, and by _Pavesi_ in “Annal. Mus. Civ. Geneva,” 1874 and 1878.

THIRD FAMILY--RHINODONTIDÆ.

_No nictitating membrane. Anal fin present. Two dorsal fins, the first nearly opposite to the ventrals, without spine in front. Mouth and nostril near the extremity of the snout._

This small family comprises one species only, _Rhinodon typicus_, a gigantic Shark, which is known to exceed a length of fifty feet, but is stated to attain that of seventy. It does not appear to be rare in the western parts of the Indian Ocean, and possibly occurs also in the Pacific. It is one of the most interesting forms, not unlike the Basking Shark of the Northern Seas, having gill-rakers like that species; but very little is known of its structure and mode of life. It is perfectly harmless, its teeth being extremely small and numerous, placed in broad bands; it has been stated to feed on tang, an observation which requires confirmation. The snout is very broad, short, and flat; the eyes are very small. A pit at the root of the caudal fin which has the lower lobe well developed; side of the tail with a keel. A characteristic figure of this fish has been given by A. Smith in his “Illustrations of the Zoology of South Africa,” Plate 26, from a specimen which came ashore at the Cape of Good Hope.

FOURTH FAMILY--NOTIDANIDÆ.

_No nictitating membrane. One dorsal fin only, without spine, opposite to the anal._

NOTIDANUS.--Dentition unequal in the jaws: in the upper jaw
one or two pairs of awl-shaped teeth, the following six being
broader, and provided with several cusps, one of which is much
the strongest. Lower jaw with six large comb-like teeth on each
side, beside the smaller posterior teeth. Spiracles small, on
the side of the neck. No pit at the root of the caudal fin.
Gill-openings wide, six in number in _Hexanchus_, seven in
_Heptanchus_.

Four species are known, distributed over nearly all the tropical and sub-tropical seas; they attain to a length of about fifteen feet. Fossil teeth belonging to this type have been found in Jurassic and later formations (_Notidanus_ and _Aellopos_).

FIFTH FAMILY--SCYLLIIDÆ.

_Two dorsal fins, without spine: the first above or behind the ventrals; anal fin present. No nictitating membrane. Spiracle always distinct. Mouth inferior. Teeth small, several series generally being in function._

SCYLLIUM.--The origin of the anal fin is always in advance of
that of the second dorsal. Nasal cavity separate from the mouth.
Teeth small, with a middle longer cusp, and generally one or two
small lateral cusps arranged in numerous series. Eggs similar to
those of the Rays (Fig. 79, p. 167).

The fishes of this genus are of small size, and commonly called “Dog-fishes.” They are coast fishes, living on the bottom, and feeding on Crustaceans, dead fishes, etc. None of the eight species known have a very wide distribution, but where they occur they are generally sufficiently abundant to prove troublesome to fishermen. They inhabit most parts of the temperate and tropical seas. On the British coasts two species are found, the “Larger” and “Lesser spotted Dog-fish,” _Scyllium canicula_ and _Scyllium catulus_, which are said to be more plentiful among the Orkney Islands than elsewhere. They are scarcely ever brought to market; but the fishermen of some localities do not disdain to eat them. Their flesh is remarkably white, a little fibrous, and dry. In the Orkneys they are skinned, split up, cleaned, and then spread out on the rocks to dry for home consumption. The skins are used for smoothing down cabinet-work. It would be worth while to apply the fins of these and other Sharks, which are so extensively used in China for making gelatine soups, to the same purpose in this country, or to dry them for exportation to the East. Most of the species of Dog-fishes are spotted, and those of the allied genera, _Parascyllium_ and _Chiloscyllium_, very handsomely ornamented.

Closely allied to _Scyllium_ is _Pristiurus_, from the coasts of Europe, which is provided with a series of small flat spines on each side of the upper edge of the caudal fin.

Fossil forms of Dog-fishes are not scarce in the Lias and Chalk: _Scylliodus_, _Palæoscyllium_, _Thyellina_, _Pristiurus_.

GINGLYMOSTOMA.--The second dorsal fin opposite to, and somewhat
in advance of, the anal. Eyes very small; spiracle minute and
behind the eye. Nasal and buccal cavities confluent. The nasal
valves of both sides form one quadrangular flap in front of the
mouth, each being provided with a free cylindrical cirrhus. The
fourth and fifth gill-openings are close together. The teeth
stand either in many series, each having a strong median cusp
and one or two smaller ones on each side (_Ginglymostoma_), or
they stand in a few (three) series only, the foremost only being
in function, and each tooth having a convex, finely and equally
serrated margin (_Nebrius_).

Four species from the tropical parts of the Atlantic and Indian Oceans, attaining to a length of some 12 feet. Pelagic.

STEGOSTOMA.--The first dorsal above the ventrals, the second in
advance of the anal, which is very close to the caudal. Tail,
with the caudal fin, exceedingly long, measuring one-half of the
total length. Eyes very small, spiracle as wide as, and situated
behind, the orbit. Nasal and buccal cavities confluent. Snout
very obtuse; upper lip very thick, like a pad, bent downwards
over the mouth, with a free cylindrical cirrhus on each side.
Teeth small, trilobed, in many series, occupying in both jaws
a transverse flat subquadrangular patch. The fourth and fifth
gill-openings are close together.

The single species (_St. tigrinum_) for which this genus has been formed, is one of the commonest and handsomest sharks of the Indian Ocean. Young individuals keep generally close to the coasts, whilst the adult, which are from 10 to 15 feet long, are not rarely met in the open ocean. The colour is a brownish yellow, ornamented with black or brown transverse bands, or with snuff-coloured rounded spots; hence this shark is frequently mentioned by the names of “Zebra-Shark” or “Tiger-Shark.”

CHILOSCYLLIUM.--The first dorsal fin above or behind the
ventrals. Anal fin placed far behind the second dorsal, and very
close to the caudal. Spiracle very distinct, below the eye.
Nasal and buccal cavities confluent. Nasal valve folded, with a
cirrhus. Teeth small, triangular, with or without lateral cusps.
The two last gill-openings close together.

“Dog-fishes,” from the Indian Ocean, of small size. Four species are known, of which one, _Ch. indicum_, is one of the commonest shore-fishes on the coasts of this region, extending from the southern extremity of the African Continent to Japan.

CROSSORHINUS.--The first dorsal behind the ventrals, the second
in advance of the anal, which is very close to the caudal. Tail
rather short. Eyes small. Spiracle a wide oblique slit, behind
and below the eye. Nasal and buccal cavities confluent. Head
broad, flat, with the snout very obtuse; mouth wide, nearly
anterior. A free nasal cirrhus; sides of the head with skinny
appendages. Anterior teeth rather large, long and slender,
without lateral lobes, the lateral tricuspid, smaller, forming
a few series only. The fourth and fifth gill-openings close
together.

Three species are known from the Australian and Japanese coasts. They are evidently ground-sharks, which lie concealed on the bottom watching for their prey. In accordance with this habit their colour closely assimilates that of a rock or stone covered with short vegetable and coralline growth--a resemblance increased by the frond-like tentacles on the side of the head. This peculiarity of the integuments, which is developed in a yet higher degree in Pediculati and Lophobranchs, is not met with in any other Selachian. These Sharks grow to a length of 10 feet.

SIXTH FAMILY--HYBODONTIDÆ.

_Two dorsal fins, each with a serrated spine. Teeth rounded, longitudinally striated, with one larger, and from two to four smaller lateral cusps. Skin covered with shagreen._

Extinct. From carboniferous, liassic, and triassic formations. Several genera have been distinguished; and if _Cladodus_ belongs to this family, it would have been represented even in the Devonian.

SEVENTH FAMILY--CESTRACIONTIDÆ.

_No nictitating membrane. Two dorsal fins, the first opposite to the space between pectoral and ventral fins; anal fin present. Nasal and buccal cavities confluent. Teeth obtuse, several series being in function._

This family is one of particular interest, because representatives of it occur in numerous modifications in primary and secondary strata. Their dentition is uniformly adapted for the prehension and mastication of crustaceous and hard-shelled animals. The fossil forms far exceeded in size the species of the only surviving genus; they make, their appearance with _Ctenoptychius_ in the Devonian; this is succeeded in the coal-measures by _Psammodus_, _Chomatodus_, _Petrodus_, _Coch__liodus_, _Polyrhizodus_, etc.; in the Trias and Chalk by _Strophodus_, _Acrodus_, _Thectodus_, and _Ptychodus_. Of the 25 genera known, 22 have lived in the periods preceding the Oolitic.

CESTRACION (HETERODONTUS).--Each dorsal fin armed with a spine
in front; the second in advance of the anal. Mouth rather
narrow. Spiracles small, below the posterior part of the eye.
Gill-openings rather narrow. Dentition similar in both jaws,
viz. small obtuse teeth in front, which in young individuals are
pointed and provided with from three to five cusps. The lateral
teeth are large, padlike, twice as broad as long, arranged in
oblique series, one series being formed by much larger teeth
than those in the other series.

Four species are known from Japan, Amboyna, Australia, the Galapagoes Islands, and California; none exceed a length of 5 feet. The egg has been figured on p. 168 (Fig. 80).

EIGHTH FAMILY--SPINACIDÆ.

_No membrana nictitans. Two dorsal fins; no anal. Mouth but slightly arched; a long, deep, straight, oblique groove on each side of the mouth. Spiracles present; gill-openings narrow. Pectoral fins not notched at their origin._

The oldest representative of this family (_Palæospinax_) occurs at Lyme Regis; its skin is granular; each dorsal fin possesses a spine; the teeth in the jaws are dissimilar--the upper being multicuspid, longitudinally ribbed as in Hybodus, the lower smooth and tricuspid. _Drepanophorus_ and _Spinax primævus_ occur in Cretaceous formations of England and the Lebanon.

CENTRINA.--Each dorsal fin with a strong spine. Trunk rather
elevated, trihedral, with a fold of the skin running along each
side of the belly. Teeth of the lower jaw erect, triangular,
finely serrated; those of the upper slender, conical, forming a
group in front of the jaw. Spiracles wide, behind the eye.

One species, _Centrina salviani_, from the Mediterranean and neighbouring parts of the Atlantic; of small size.

ACANTHIAS.--Each dorsal fin with a spine. Teeth equal in both
jaws, rather small; their point is so much turned aside that
the inner margin of the tooth forms the cutting edge. Spiracles
rather wide, immediately behind the eye.

The two species of “Spiny Dog-fishes,” _A. vulgaris_ and _A. blainvillii_, have a very remarkable distribution, being found in the temperate seas of the Northern and Southern Hemispheres, but not in the intermediate tropical zone. They are of small size, but occur at times in incredible numbers, 20,000 having been taken in one scene on the Cornish coast. They do much injury to the fishermen by cutting their lines and carrying off their hooks.

CENTROPHORUS.--Each dorsal fin with a spine which, however, is
sometimes so small as to be hidden below the skin. Mouth wide.
Teeth of the lower jaw with the point more or less inclined
backwards and outwards. Upper teeth erect, triangular, or
narrow, lanceolate, with a single cusp. Spiracles wide, behind
the eye.