round the hinder extremity of the body. The skeleton remains throughout life in a cartilaginous condition, the chorda dorsalis is persistent, and the only traces of bodies of vertebræ are found in hardly perceptible rings of osseous matter developed in the sheath of the notochord. The neural arches of the vertebræ, enclosing the spinal cord, are only represented by cartilaginous prolongations. The mouth in the Hag-fish (Myxine) is of a very remarkable character, and enables it to lead a very peculiar mode of life. It is usually found, namely, embedded in the interior of some other larger fish, into which it has succeeded in penetrating by means of its singular dental apparatus. The mouth is sucker-like, destitute of jaws, but provided with tactile filaments or cirri. In the centre of the palate is fixed a single, large, recurved fang, which is firmly attached to the under surface of the cranium. The sides of this fang are strongly serrated, and it is by means of this that the Hag-fish bores its way into its victim, having previously attached itself by its sucker-like mouth. In the Lampreys the mouth has also the form of a circular cup or sucker, and is also destitute of jaws; but in addition to the palatine fang of the Myxine, the margins of the lips bear a number of horny processes, which are not really true teeth, but are hard structures developed in the labial mucous membrane. The tongue, also, is armed with serrated teeth, and acts as a kind of piston; so that the Lampreys are in this manner enabled to attach themselves firmly to solid objects.

A very remarkable peculiarity in the Hag-fishes, and one very necessary to remember, is found in the structure of the nasal sacs. In all fishes, namely, except these and the Mudfishes (Lepidosiren), the nasal sacs are closed behind, and do not open posteriorly into the throat. In the Myxinoids, however, such a communication exists, and the nasal sac-for there is only one-is placed in communication with the cavity of the mouth by means of a canal which perforates the palate. In front the nasal cavity communicates with the external medium by a second tube, which opens on the top of the head by a single aperture, which is often called the "spiracle,” and which is in reality an unpaired nostril. In the Lampreys, on the other hand, the single nasal sac has the same structure as in the typical fishes-that is to say, it is closed behind, and does not communicate in any way with the cavity of the mouth.

Another very remarkable point in the Hag-fishes and Lampreys is to be found in the structure of the gills, from which the name of the order is derived. In the Lampreys, in place

of the single gill-slit, covered by a gill-cover, as seen in the ordinary bony fishes, the side of the neck, when viewed externally, exhibits six or seven round holes placed far back in a line on each side (fig. 133, A). In the Hag-fishes the external apertures of the gills are reduced to one on each side, placed below the head; but the internal structure of the gills is the same in both cases. In both the Lampreys and the Hag-fishes, namely, the gills are in the form of sacs or pouches (fig. 133, B), the mucous membrane of which is thrown into folds or plaits like the leaves of a book, over which the branchial vessels ramify. Internally the sacs communicate with the cavity of the pharynx, either directly or by the intervention of a common respiratory tube. It follows from this, that the gill-pouches on the two sides, with their included fixed branchial laminæ, communicate freely with one another through the pharynx. The object of this arrangement appears to be mainly that of obviating the necessity of admitting water to the gills through the mouth, as is the case with the ordinary bony fishes. These fishes are in the habit of fixing themselves to foreign objects by means of the suctorial mouth; and when in this position it is, of course, impossible that they can obtain the necessary water of respiration through the mouth. As the branchial pouches, however, on the two sides of the neck communicate freely with one another through the pharynx, water can readily pass in and out. This, in the Lampreys, is further assisted by a kind of elastic cartilaginous framework upon which the respiratory apparatus is supported, and which acts somewhat like the ribs of the higher Vertebrata. Water can also be admitted to the pharynx, and thence to the branchial sacs, by means of a tube which leads from the pharynx to an aperture placed on the top of the head.

The Lampreys are, some of them, inhabitants of rivers; but the great Sea-lamprey (Petromyzon marinus) only quits the salt water in order to spawn. The Hag-fish (Myxine glutinosa) is an inhabitant of the North Sea, and is commonly captured on the Norwegian coast.

CHAPTER LVI.

TELEOSTEI.

ORDER III. TELEOSTEI.-This order includes the great majority of fishes in which there is a well-ossified endoskeleton,

and it corresponds very nearly with Cuvier's division of the "osseous" fishes. The Teleostei are defined as follows :-The skeleton is usually well ossified; the cranium is provided with cranial bones; and a mandible is present; whilst the vertebral column almost always consists of more or less completely ossified vertebræ. The pectoral arch has a clavicle; and the two pairs of limbs, when present, are in the form of fins supported by rays. The gills are free, pectinated or tufted in shape; a bony gill-cover and branchiostegal rays being always developed. The branchial artery has its base developed into a bulbus arteriosus; but this is never rhythmically contractile, and is separated from the ventricle by no more than a single row of valves.

The order Teleostei comprises almost all the common fishes; and it will be unnecessary to dilate upon their structure, as they were taken as the types of the class in giving a general description of the Fishes. It may be as well, however, to recapitulate very briefly some of the leading characters of the order.

I. The skeleton, instead of remaining throughout life more or less completely cartilaginous, is now always more or less thoroughly ossified. The notochord is not persistent, and the vertebral column, though sometimes cartilaginous, consists of a number of vertebræ. The bodies of the vertebræ are what is called "amphicœlous"- that is to say, they are concave at both ends. It follows from this, that between each pair of vertebræ there is formed a doubly-conical cavity, and this is filled with the cartilaginous or semi-gelatinous remains of the notochord. By this means an extraordinary amount of flexibility is given to the entire vertebral column. In no fish except the Bony Pike (which belongs to the order Ganoidei) is the ossification of the vertebral centra carried further than this. The skull is of an extremely complicated nature, being composed of a number of distinct cranial bones; and a mandible or lower jaw is invariably present.

II. The anterior and posterior pairs of limbs are usually, but not always, present, and when developed they are always in the form of fins. The fins may be supported by "spinous" or "soft" rays, of which the former are simple undivided spines of bone, whilst the latter are divided transversely into a number of short transverse pieces, and also are broken up into a number of longitudinal rays proceeding from a common root. (The Fishes with soft rays in their paired fins are termed "Malacopterygii"-those with spinous rays, "Acanthopterygii.") III. Besides the paired fins, representing the limbs, there is

a variable number of unpaired or azygous integumentary expansions, which are known as the "median fins." When fully developed (fig. 129), they consist of one or two fins on the back-the "dorsal" fins; one or two on the ventral surfacethe "anal" fins; and one clothing the posterior extremity of the body-the "caudal" fin. The caudal fin is set vertically, and not horizontally, as in the Whales and Dolphins; and in all the bony fishes its form is "homocercal"—that is, it consists of two equal lobes, and the vertebral column is not prolonged into the superior lobe. In all the median fins the fin-rays are supported upon a series of dagger-shaped bones, which are plunged in the flesh of the middle line of the body, and are attached to the spinous processes of the vertebræ. These are the so-called "interspinous" bones.

IV. The heart consists of two chambers-an auricle and a ventricle, and the branchial artery is furnished with a bulbus arteriosus. The arterial bulb, however, is not furnished with a special coat of striated muscular fibres, is not rhythmically contractile, and is separated from the ventricle by no more than a single row of valves (fig. 134, A).

Fig. 134-A, Heart of the Angler (Lophius piscatorius). B, Arterial bulb of Bony Pike (Lepidosteus) cut open. C, Heart of the saine, viewed externally: a Auricle; Ventricle; b Arterial bulb.

V. The respiratory organs consist of free, pectinated, or tufted branchiæ, situated in two branchial chambers, each of which communicates internally with the pharynx by a series of clefts, and opens externally on the side of the neck by a single aperture (or "gill-slit "), which is protected in front by a bony gillcover, and is also closed by a "branchiostegal membrane," supported upon "branchiostegal rays." The branchia are attached to a series of bony branchial arches, which are connected inferiorly with the hyoid bone and superiorly with the skull; and

the water required in respiration is taken in at the mouth by a process analogous to swallowing.

VI. The nasal sacs never communicate posteriorly with the cavity of the pharynx.

The subdivisions of the osseous fishes are so numerous, and they contain so many families, that it will be sufficient to run over the more important sub-orders, and to mention the more familiar examples of each.

SUB-ORDER A. MALACOPTERI, Owen (= Physostomata, Müller). This sub-order is defined by usually possessing a complete set of fins, supported by rays, all of which are "soft" or many-jointed, with the occasional exception of the first rays in the dorsal and pectoral fins. A swim-bladder is always present, and always communicates with the oesophagus by means of a duct, which is the homologue of the windpipe. The skin is rarely naked, and is mostly furnished with cycloid scales; but in some cases ganoid plates are present.

This sub-order is one of great importance, as comprising many well-known and useful fishes. It is divided into two groups, according as ventral fins are present or not. In the first group-Apoda-there are no ventral fins; and the most familiar examples are the common Eels of our own country. More remarkable, however, than the ordinary Eels is the Gymnotus electricus, or great Electric Eel, which inhabits the marshy waters of those wonderful South American plains, the so-called "Llanos." This extraordinary fish (fig. 13.5) is from

Fig. 135.- Electric Eel (Gymnotus electricus).

five to six feet in length, and the discharge of its electrical organs is sufficiently powerful to kill even large animals. The following striking account is given by Humboldt of the manner in which the Gymnoti are captured by the Indians :—“ A number of horses and mules are driven into a swamp which is closely surrounded by Indians, until the unusual disturbance excites the daring fish to venture an attack. Serpent-like,