Ophichthys.—Nostrils labial; extremity of the tail free, not surrounded by a fin.
More than eighty species are known, many of which are abundant on the coasts of the tropical and sub-tropical zones. They do not attain to a large size, but many must be extremely voracious and destructive to other fishes, if we draw an inference from the formidable dentition with which their jaws and palate is armed. Other species have much more feeble, and some even obtuse teeth, better adapted for seizing Crustaceans than vigorous and slippery fishes. Some have rudimentary pectoral fins or lack them altogether. Many are highly ornamented with bands or spots, the coloration being apparently very constant in the several species.
Fig. 303.—Ophichthys crocodilinus, from the Indo-Pacific.
Moringua.—Body scaleless, cylindrical, with the trunk much longer than the tail. Pectorals none or small; vertical fins but little developed, limited to the tail. Posterior nostrils in front of the small eye. Cleft of the mouth narrow; teeth uniserial. Heart placed far behind the branchiæ. Gill-openings rather narrow, inferior.
Six species from freshwaters, brackish water, and the coasts of India to the Fiji Islands.
Muræna.—Scaleless. Teeth well developed. Gill-openings and clefts between the branchial arches narrow. Pectoral fins none; dorsal and anal fins well developed. Two nostrils on each side of the upper surface of the snout; the posterior a narrow round foramen, with or without tube; the anterior in a tube.
Fig. 304.—Head of a Muræna.
Fig. 305.—Muræna pavonina, from Southern Seas.
The Murænas are as abundantly represented in the tropical and sub-tropical zones, and have nearly the same range, as Ophichthys. The number of species known exceeds eighty. The majority are armed with formidable pointed teeth, well suited for seizing other fish on which they prey. Large specimens thus armed readily attack persons in and out of the water; and as some species attain a length of some six or eight feet, they are justly feared by fishermen. The minority of species have obtuse and molar-like teeth, their food consisting chiefly of Crustaceans and other hard-shelled animals. Most of the Murænas are beautifully coloured and spotted, some in a regular and constant manner, whilst in others the pattern varies in a most irregular fashion: they have quite the appearance of snakes. The Muræna of the Ancient Romans is Muræna helena, which is not confined to the Mediterranean, but also found in the Indian Ocean and on the coast of Australia. Its skin is of a rich brown, beautifully marked with large yellowish spots, each of which contains smaller brown spots.
Fig. 306.—Muræna picta, from the Indo-Pacific.
Gymnomuræna differs from Muræna in having the fins reduced to a short rudiment near the end of the tail. Six species are known growing to a length of eight feet.
Fig. 307.—Gymnomuræna vittata, from Cuba.
Myroconger and Enchelycore belong to the same sub-family as Muræna, but the former is provided with pectoral fins, and in the latter the posterior nostril is a long slit, and not round as in the other genera.
The gills are not laminated, but composed of small rounded lobes attached to the branchial arches. Gill-cover reduced to a large simple plate. Air-bladder simple, without pneumatic duct. A dermal skeleton composed of numerous pieces arranged in segments, replaces more or less soft integuments. Muscular system not much developed. Snout prolonged. Mouth terminal, small, toothless, formed as in Acanthopterygians.
Fig. 308.—Gills of Hippocampus abdominalis.
Gill-openings wide. Two dorsal fins, the rays of the anterior not articulated. All the other fins well developed.
One living genus only is known, which was preceded in the tertiary epoch by Solenorhynchus (Monte Postale).
Solenostoma.—Snout produced into a long tube. Body compressed, with very short tail. All parts covered with thin skin, below which there is a dermal skeleton formed by large star-like ossifications. The soft dorsal and anal fins on elevated bases; caudal fin long. Ventral fins inserted opposite to the anterior dorsal, close together, seven-rayed; they are free in the male, but in the female their inner side coalesces with the integuments of the body, a large pouch for the reception of the eggs being formed thereby. Air-bladder and pseudobranchiæ absent. Branchiostegals four, very thin. Intestinal tract very simple, with a stomachic dilatation, without pyloric appendages. Ova very small.
The dermal skeleton of this singular type is formed by star-like ossifications, four in each horizontal and vertical series on the side of the fore part of the trunk; each consists of four or three radiating branches by which it joins the neighbouring bones; on the hind part of the trunk and tail the series are diminished to two. The dorsal and abdominal profiles in front of the fins are protected by similar bones. The vertebral column is composed of eighteen abdominal and fifteen caudal vertebræ, the vertebræ gradually decreasing in length backwards, so that the shortness of the tail is caused not only by the smaller number of vertebræ, but also by their much lesser length. Neural and hæmal spines are developed. The pelvis consists of two pairs of cartilaginous laminæ, the convex margin of the anterior fitting into an angle of a dermal bone which separates the pelvis from the well-ossified humeral arch.
The singular provision for the retention and protection of the eggs has been described above (p. 162, figs. 73 and 74), and we have only to repeat here that it is the female which takes care of the progeny, and not the male as in the following family. Two or three small species are known from the Indian Ocean; they are beautifully marked, especially the male, which also appears to be of smaller size in this genus than the female.
Gill-openings reduced to a very small opening near the upper posterior angle of the gill-cover. One soft dorsal fin; no ventrals, and, sometimes, one or more of the other fins are also absent.
Small marine fishes, which are abundant on such parts of the coasts of the tropical and temperate zones as offer by their vegetation shelter to these defenceless creatures. They are bad swimmers (the dorsal fin being the principal organ of locomotion), and frequently and resistlessly carried by currents into the open ocean or to distant coasts. All enter brackish water, some fresh water. The strata of Monte Bolca and Licata (Sicily) have, yielded evidence of their existence in the tertiary epochs; beside species of Siphonostoma and Syngnathus (Pseudosyngnathus), remains of an extinct genus, Calamostoma, allied to Hippocampus, but with a distinct caudal fin, have been found. On their propagation see p. 163, Fig. 76.
A. Syngnathina.—The tail is not prehensile, and generally provided with a caudal fin.—Pipe-Fishes.
Siphonostoma.—Body with distinct ridges, the upper caudal ridge continuous with the lateral line, but not with the dorsal ridge of the trunk. Pectoral and caudal fins well developed; dorsal fin of moderate length, opposite to the vent. Humeral bones movable, not united into a “breast-ring.” Males with an egg-pouch on the tail, the eggs being covered by cutaneous folds.
Two species, of which S. typhle is common on the British, and generally distributed on the European coasts.
Syngnathus.—Body with the ridges more or less distinct, the dorsal ridge of the trunk not being continuous with that of the tail. Pectoral fins well developed; caudal present. Dorsal fin opposite or near to the vent. Humeral bones firmly united into the breast-ring. Egg-pouch as in Siphonostoma.
The distribution of this genus nearly coincides with that of the family, some fifty species being known. S. acus, the great Pipe-fish (see Fig. 75, p. 163), is one of the most common European fishes, extending across the Atlantic and southwards to the Cape of Good Hope; it attains a length of 18 inches. Another very common species, frequently met at sea, and spread over nearly all the tropical and sub-tropical seas, is S. pelagicus, agreeably marked with alternate brown and silvery cross-bars.
Doryichthys.—Body with the ridges well developed. Pectoral and caudal fins present. Dorsal fin long or of moderate length, opposite to the vent. Humeral bones firmly united. Males with the lower ridges of the abdomen dilated, the dilated parts forming a broad groove for the reception of the ova.
In these Pipe-fishes the ova are not received in a completely closed pouch, but glued on to the surface of the abdomen. Twenty species from tropical seas.
Nerophis.—Body smooth, rounded, with scarcely any of the ridges distinct. Pectoral fin none, caudal absent or rudimentary, the tail tapering into a point. Dorsal fin of moderate length, opposite to the vent. The ova are attached to the soft integument of the abdomen of the male, and are not covered by lateral folds of the skin.
Seven species from the European seas and the Atlantic. N. æquoreus (Ocean Pipe-fish), N. ophidion (Straight-nosed Pipe-fish), and N. lumbriciformis (Little Pipe-fish), are common on the British coasts.
Protocampus.—The whole dermal skeleton is covered with skin. A broad cutaneous fold runs along the back in front and behind the dorsal; a similar fold along the abdomen. Pectoral fin none; caudal very small.
The single species of this remarkable genus, P. hymenolomus, occurs in the Falkland Islands. It may be regarded as an embryonal form of Nerophis, the median skin-folds being evidently remains of the fringe which surrounds the body of the embryo.
The other genera belonging to this group are, Icthyocampus, Nannocampus, Urocampus, Leptoichthys, Coelonotus, and Stigmatophora.
Hippocampina.—The tail is prehensile, and invariably without caudal fin.—Sea-horses.
Gastrotokeus.—Body depressed, the lateral line running along the margin of the abdomen. Shields smooth. Tail shorter than the body. Pectoral fins. No pouch is developed for the ova, which are imbedded in the soft integument of the abdomen of the male.
Gastrotokeus biaculeatus, very common in the Indian Ocean to the coasts of Australia.
Solenognathus.—Body compressed, deeper than broad. Shields hard, rugose, with round or oval interannular plates; and without elongate processes. Tail shorter than the body. Pectoral fins.
Three species, from the Chinese and Australian Seas; they are the largest of Lophobranchs, S. hardwickii, attaining to a length of nearly two feet.
Fig. 309.—Phyllopteryx eques.
Phyllopteryx.—Body compressed, or as broad as deep. Shields smooth, but some or all of them are provided with prominent spines or processes on the edges of the body; some of the processes with cutaneous filaments. A pair of spines on the upper side of the snout and above the orbit. Tail about as long as the body. Pectoral fins. The ova are imbedded in soft membrane on the lower side of the tail, without a pouch being developed.
Three species from the coasts of Australia. The protective resemblances with which many Lophobranchs are furnished, attain to the highest degree of development in the fishes of this genus. Not only their colour closely assimilates that of the particular kind of seaweed which they frequent, but the appendages of their spines seem to be merely part of the fucus to which they are attached. They attain a length of 12 inches.
Hippocampus.—Trunk compressed, more or less elevated. Shields with more or less prominent tubercles or spines. Occiput compressed into a crest, terminating at its supero-posterior corner in a prominent knob (coronet). Pectoral fins. The males carry the eggs in a sac at the base of the tail, opening near the vent.
A singular resemblance of the head and fore part of the body to that of a horse, has given to these fishes the name of “Sea-horses.” They are abundant between and near the tropics, becoming scarcer in higher latitudes. Some twenty species are known, some of which have a wide geographical range, as they are often carried to great distances with floating objects to which they happen to be attached.—Acentronura is a genus closely allied to Hippocampus.
Teleosteous fishes with rough scales, or with ossifications of the cutis in the form of scutes or spines; skin sometimes entirely naked. Skeleton incompletely ossified, with the vertebræ in small number. Gills pectinate; a narrow gill-opening in front of the pectoral fins. Mouth narrow; the bones of the upper jaw generally firmly united. A soft dorsal fin, belonging to the caudal portion of the vertebral column, opposite to the anal; sometimes elements of a spinous dorsal besides. Ventral fin none, or reduced to spines. Air-bladder without pneumatic duct.
Snout somewhat produced; jaws armed with distinct teeth in small number. Skin with scutes or rough. The elements of a spinous dorsal and ventral fins generally present.
Marine fishes of moderate or small size, very common in the tropical zone, but scarcer in higher latitudes. They have been found in three localities of tertiary strata, viz., at Monte Bolca, where a species of Ostracion occurs, and in the Schists of Glaris, from which two genera have been described, Acanthoderma and Acanthopleurus, closely allied to Balistes and Triacanthus. Glyptocephalus from the Isle of Sheppey has the skull of a Balistes, but its body is covered with tubercles arranged in regular series. The Scleroderms may be divided into three very natural groups:—
A. Triacanthina.—The skin is covered with small, rough, scale-like scutes. A spinous dorsal fin with from four to six spines. A pair of strong, movable ventral spines, joined to the pelvic bone.
To this group belong the genera Triacanthodes, Hollardia, and Triacanthus, represented by five species, of which Triacanthus brevirostris from the Indian Ocean is the most common.
B. Balistina.—Body compressed, covered with movable scutes or rough. Spinous dorsal reduced to one, two, or three spines. Ventral fins reduced to a single pelvic prominence, or entirely absent.
To this group belong the genera Balistes, Monacanthus, and Anacanthus, the last genus being distinguished by a barbel at the lower jaw.
Fig. 310.—Balistes vidua.
Balistes, or the “File-fishes” proper, inhabit the tropical and sub-tropical seas; shoals of young are not rarely met with in mid-ocean. Some thirty species are known, many attaining a length exceeding two feet; but the majority are much smaller, and frequently beautifully and symmetrically marked. Both jaws are armed with eight strong incisor-like and obliquely truncated teeth, by which these fishes are enabled to break off pieces of corals on which they feed, or to chisel a hole into the hard shell of Mollusca, in order to extract the soft parts. They destroy an immense number of Mollusks, thus becoming most injurious to the pearl-fisheries. The first of their three dorsal spines is very strong, roughened in front like a file, and hollowed out behind to receive the second much smaller spine, which, besides, has a projection in front, at its base, fitting into a notch of the first. Thus these two spines can only be raised or depressed simultaneously, and the first cannot be forced down, unless the second has been previously depressed. The latter has been compared to a trigger, hence a second name, “Trigger-fish,” has been given to these fishes. Some species are armed with a series of short spines or tubercles on each side of the tail. Two species (B. maculatus and B. capriscus), common in the Atlantic, sometimes wander to the British coasts.
The Monacanthus are similarly distributed as the Balistes, and still more abundant, some fifty species being known. Their dentition is very similar, but they possess one dorsal spine only, and their rough scales are so small as to give a velvety appearance to the skin (Figs. 17 and 18, p. 48). Adult males of some of the species possess a peculiar armature on each side of the tail, which in females is much less developed or entirely absent. This armature may consist either in simple spines arranged in rows, or in the development of the minute spines of the scales into long stiff bristles, so that the patch on each side of the tail looks like a brush.
C. Ostraciontina.—The integuments of the body form a hard continuous carapace, consisting of hexagonal scutes juxtaposed in mosaic-fashion. A spinous dorsal and ventral fins are absent; but sometimes indicated by protuberances.
The “Coffer-fishes” (Ostracion) are too well known to require a lengthened description. Only the snout, the bases of the fins, and the hind part of the tail are covered with soft skin, so as to admit of free action of the muscles moving these parts. The mouth is small, the maxillary and intermaxillary bones coalescent, each jaw being armed with a single series of small slender teeth. The short dorsal fin is opposite to the equally short anal. The vertebral column consists of fourteen vertebræ only, of which the five last are extremely short, the anterior elongate. Ribs none. The carapaces of some species are three-ridged, of others four- and five-ridged, of some provided with long spines. Twenty-two species from tropical and sub-tropical seas are known.
Body more or less shortened. The bones of the upper and lower jaw are confluent, forming a beak with a trenchant edge, without teeth, with or without median suture. A soft dorsal, caudal and anal are developed, approximate. No spinous dorsal. Pectoral fins; no ventrals.
Marine fishes of moderate or small size from tropical and sub-tropical seas. A few species live in fresh water. Fossil remains of Diodon are not scarce at Monte Bolca and Licata; a distinct genus, Enneodon, has been described from Monte Postale. The Gymnodonts may be divided into three groups:
A. Triodontina.—Tail rather long, with a separate caudal fin. Abdomen dilatable into a very large, compressed, pendent sac, the lower part of which is merely a flap of skin, into which the air does not penetrate, the sac being capable of being expanded by the very long pelvic bone. The upper jaw divided by a median suture, the lower simple.
A single genus and species (Triodon bursarius) from the Indian Ocean.
B. Tetrodontina.—Tail and caudal fin distinct. Part of the œsophagus much distensible, and capable of being filled with air. No pelvic bone.
“Globe-fishes” have a short, thick, cylindrical body, with well developed fins. It is covered with thick scaleless skin, in which, however, spines are imbedded of various sizes. The spines are very small, and but partially distributed over the body in some species, whilst in others they are very large, and occupy equally every part of the body. These fishes have the power of inflating their body by filling their distensible œsophagus with air, and thus assume a more or less globular form. The skin is, then, stretched to its utmost extent, and the spines protrude and form a more or less formidable defensive armour, as in a hedgehog; therefore they are frequently called “Sea-hedgehogs.” A fish thus blown out turns over and floats belly upwards, driving before the wind and waves. However, it is probable that the spines are a protection not only when the fish is on the surface and able to take in air, but also when it is under water. Some Diodonts, at any rate, are able to erect the spines about the head by means of cutaneous muscles; and, perhaps, all fill their stomach with water instead of air, for the same purpose and with the same effect. In some Diodonts the spines are fixed, erect, not movable. The Gymnodonts generally, when taken, produce a sound, doubtless by the expulsion of air from the œsophagus. Their vertebral column consists of a small number of vertebræ, from 20 to 29, and their spinal chord is extremely short. All these fishes have a bad reputation, and they are never eaten; indeed, some of them are highly poisonous, and have caused long continued illness and death. Singularly, the poisonous properties of these fishes vary much as regards intensity, only certain individuals of a species, or individuals from a certain locality, or caught at a certain time of the year, being dangerous. Therefore it is probable that they acquire their poisonous quality from their food, which consists in corals and hard-shelled Mollusks and Crustaceans. Their sharp beaks, with broad masticating posterior surface, are admirably adapted for breaking off branchlets of coral-stocks, and for crushing hard substances.
Fig. 311.—Jaws of Tetrodon.
Fig. 312.—Tetrodon margaritatus.
Tetrodon (including Xenopterus).—Both the upper and lower jaws are divided into two by a mesial suture.
Extremely numerous in tropical and sub-tropical zones, more than sixty species being known. In some of the species the dermal spines are extremely small, and may be absent altogether. Many are highly ornamented with spots or bands. A few species live in large rivers—thus T. psittacus from Brazil; T. fahaka, a fish well known to travellers on the Nile, and likewise abundant in West African rivers; T. fluviatilis from brackish water and rivers of the East Indies. The species figured is one of the smallest, about six inches long, and common in the Indo-Pacific.
Diodon.—Jaws without mesial suture, so that there is only one undivided dental plate above and one below.
In these fishes, as well as in some closely allied genera, the dermal spines are much more developed than in the Tetrodonts; in some the spines are erectile, as in Diodon, Atopomycterus, Trichodiodon, and Trichocyclus; in others they are stiff and immovable, as in Chilomycterus and Dicotylichthys. Seventeen species are known, of which Diodon hystrix is the most common as it is the largest, growing to a length of two feet. It is spread over the Tropical Atlantic as well as Indo-Pacific, as is also a smaller, but almost equally common species, Diodon maculatus.
Fig. 313.—Diodon maculatus.
Fig. 314.—Diodon maculatus, inflated.
C. Molina.—Body compressed, very short; tail extremely short, truncate. Vertical fins confluent. No pelvic bone.
The “Sun-fishes” (Orthagoriscus) are pelagic fishes, found in every part of the oceans within the tropical and temperate zones. The singular shape of their body and the remarkable changes which they undergo with age, have been noticed above (p. 175, Figs. 93, 94). Their jaws are undivided in the middle, comparatively feeble, but well adapted for masticating their food, which consists of small pelagic Crustaceans. Two species are known. The common Sun-fish, O. mola, which attains to a very large size, measuring seven or eight feet, and weighing as many hundredweights. It has a rough, minutely granulated skin. It frequently approaches the southern coasts of England and the coasts of Ireland, and is seen basking in calm weather on the surface. The second species, O. truncatus, is distinguished by its smooth, tessellated skin, and one of the scarcest fishes in collections. The shortness of the vertebral column of the Sun-fishes, in which the number of caudal vertebra is reduced to seven, the total number being seventeen, and the still more reduced length of the spinal chord have been noticed above (p. 96).
Skeleton cartilaginous and notochordal, without ribs and without real jaws. Skull not separate from the vertebral column. No limbs. Gills in the form of fixed sacs, without branchial arches, six or seven in number on each side. One nasal aperture only. Heart without bulbus arteriosus. Mouth anterior, surrounded by a circular or subcircular lip, suctorial. Alimentary canal straight, simple, without coecal appendages, pancreas or spleen. Generative outlet peritoneal. Vertical fins rayed.
The Cyclostomes are most probably a very ancient type. Unfortunately the organs of these creatures are too soft to be preserved, with the exception of the horny denticles with which the mouth of some of them is armed. And, indeed, dental plates, which are very similar to those of Myxine, are not uncommon in certain strata of Devonian and Silurian age (see p. 193). The fishes belonging to this sub-class may be divided into two families—
Body eel-shaped, naked. Subject to a metamorphosis; in the perfect stage with a suctorial mouth armed with teeth, simple or multicuspid, horny, sitting on a soft papilla. Maxillary, mandibulary, lingual, and suctorial teeth may be distinguished. Eyes present (in mature animals). External nasal aperture in the middle of the upper side of the head. The nasal duct terminates without perforating the palate. Seven branchial sacs and apertures on each side behind the head; the inner branchial ducts terminate in a separate common tube. Intestine with a spiral valve. Eggs small. The larvæ without teeth, and with a single continuous vertical fin.
“Lampreys” are found in the rivers and on the coasts of the temperate regions of the northern and southern hemispheres. Their habits are but incompletely known, but so much is certain that at least some of them ascend rivers periodically, for the purpose of spawning, and that the young pass several years in rivers, whilst they undergo a metamorphosis (see p. 170). They feed on other fishes, to which they suck themselves fast, scraping off the flesh with their teeth. Whilst thus engaged they are carried about by their victim; Salmon have been captured in the middle course of the Rhine with the Marine Lamprey attached to them.
Fig. 315.—Mouth of Larva of Petromyzon branchialis.
Fig. 316.—Mouth of Petromyzon fluviatilis. mx, Maxillary tooth; md, Mandibulary tooth; l, Lingual tooth; s, Suctorial teeth.
Petromyzon.—Dorsal fins two, the posterior continuous with the caudal. The maxillary dentition consists of two teeth placed close together, or of a transverse bicuspid ridge; lingual teeth serrated.
The Lampreys belonging to this genus are found in the northern hemisphere only; the British species are the Sea-Lamprey (P. marinus), exceeding a length of three feet, and not uncommon on the European and North American coasts; the River-Lamprey or Lampern (P. fluviatilis), ascending in large numbers the rivers of Europe, North America, and Japan, and scarcely attaining a length of two feet; the “Pride” or “Sand-Piper” or Small Lampern (P. branchialis), scarcely twelve inches long, the larva of which has been long known under the name of Ammocoetes.
Ichthyomyzon from the western coasts of North America is said to have a tricuspid maxillary tooth.
Mordacia.—Dorsal fins two, the posterior continuous with the caudal. The maxillary dentition consists of two triangular groups, each with three conical acute cusps; two pairs of serrated lingual teeth.
Fig. 317.—Mouth of Mordacia mordax, closed and opened.
A Lamprey (M. mordax) from the coasts of Chile and Tasmania. This fish seems to be provided sometimes with a gular sac, like the following.[47]
Fig. 318.—Mordacia mordax.
Geotria.—Dorsal fins two, the posterior separate from the caudal. Maxillary lamina with four sharp flat lobes; a pair of long pointed lingual teeth.
Two species, one from Chile and one from South Australia. They grow to a length of two feet, and in some specimens the skin of the throat is much expanded, forming a large pouch. Its physiological function is not known. The cavity is in the subcutaneous cellular tissue, and does not communicate with the buccal or branchial cavities. Probably it is developed with age, and absent in young individuals. In all the localities in which these Extra-european Lampreys are found, Ammocoetes forms occur, so that there is little doubt that they undergo a similar metamorphosis as P. branchialis.
Body eel-shaped, naked. The single nasal aperture is above the mouth, quite at the extremity of the head, which is provided with four pairs of barbels. Mouth without lips. Nasal duct without cartilaginous rings, penetrating the palate. One median tooth on the palate, and two comb-like series of teeth on the tongue (see Fig. 101). Branchial apertures at a great distance from the head; the inner branchial ducts lead into the œsophagus. A series of mucous sacs along each side of the abdomen. Intestine without spiral valve. Eggs large, with a horny case provided with threads for adhesion.
Fig. 319.—Ovum of Myxine glutinosa, enlarged.
The fishes of this family are known by the names of “Hag-Fish,” “Glutinous Hag,” or “Borer;” they are marine fishes with a similar distribution as the Gadidæ, being most plentiful in the higher latitudes of the temperate zones of the northern and southern hemispheres. They are frequently found buried in the abdominal cavity of other fishes, especially Gadoids, into which they penetrate to feed on their flesh. They secrete a thick glutinous slime in incredible quantities, and are therefore considered by fishermen a great nuisance, seriously damaging the fisheries and interfering with the fishing in localities where they abound. Myxine descends to a depth of 345 fathoms, and is generally met with in the Norwegian Fjords at 70 fathoms, sometimes in great abundance.
Myxine.—One external branchial aperture only on each side of the abdomen, leading by six ducts to six branchial sacs.
Three species from the North Atlantic, Japan, and Magelhæn’s Straits.
Fig. 320.—Myxine australis. A, Lower aspect of head; a, Nasal aperture; b, Mouth; g, Branchial aperture; v, Vent.
Bdellostoma.—Six or more external branchial apertures on each side, each leading by a separate duct to a branchial sac.
Two species from the South Pacific.
Skeleton membrano-cartilaginous and notochordal, ribless. No brain. Pulsating sinuses in place of a heart. Blood colourless. Respiratory cavity confluent with the abdominal cavity; branchial clefts in great number, the water being expelled by an opening in front of the vent. Jaws none.
This sub-class is represented by a single family (Cirrostomi) and by a single genus (Branchiostoma);[48] it is the lowest in the scale of fishes, and lacks so many characteristics, not only of this class, but of the vertebrata generally, that Hæckel, with good reason, separates it into a separate class, that of Acrania. The various parts of its organisation have been duly noticed in the first part of this work.
The “Lancelet” (Branchiostoma lanceolatum, see Fig. 28, p. 63), seems to be almost cosmopolitan within the temperate and tropical zones. Its small size, its transparency, and the rapidity with which it is able to bury itself in the sand, are the causes why it escapes so readily observation, even at localities where it is known to be common. Shallow, sandy parts of the coasts seem to be the places on which it may be looked for. It has been found on many localities of the British, and generally European coasts, in North America, the West Indies, Brazil, Peru, Tasmania, Australia, and Borneo. It rarely exceeds a length of three inches. A smaller species, in which the dorsal fringe is distinctly higher and rayed, and in which the caudal fringe is absent, has been described under the name of Epigionichthys pulchellus; it was found in Moreton Bay.