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

VII. The exoskeleton usually has the form of overlapping horny scales of the cycloid or ctenoid character; but it is sometimes absent, sometimes composed of scattered plates of true bone, sometimes ganoid, and sometimes formed of shagreen-like bony spines.

VIII. The stomach is capacious; pyloric cæca are present; the intestine has no spiral valve; and the rectum usually opens separate from and in front of the urinary and genital apertures. The air-bladder may or may not be present, and may or may not communicate with the gullet. The kidneys are well developed. The reproductive organs may be solid, and may liberate their contents by rupture into the abdominal cavity; but they are usually hollow organs, with ducts which open beside or behind the urinary aperture.

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 wellknown 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. The Eels (Murænida) have an elongated, almost cylindrical body, with the scales deeply sunk in the skin, and scarcely apparent. A swim-bladder is present, and the operculum is small and mostly enveloped in the skin. 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. 242) is from 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, they are seen swimming along the surface of the water, striving cunningly to glide under the bellies of the horses. By the force of their invisible blows numbers of the poor animals

are suddenly prostrated; others, snorting and panting, their manes erect, their eyes wildly flashing terror, rush madly from the raging storm; but the Indians, armed with long bamboo staves, drive them back into the midst of the pool.

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'By degrees the fury of this unequal contest begins to slacken. Like

Fig. 242.- Electric Eel (Gymnotus electricus).

clouds which have discharged their electricity, the wearied eels disperse. They require long rest and nourishing food to repair the galvanic force which they have so lavishly expended. Their shocks gradually become weaker and weaker. Terrified by the noise of the trampling horses, they timidly approach the banks of the morass, where they are wounded by harpoons, and drawn on shore by non-conducting pieces of dry wood.

Such is the remarkable contest between horses and fish. That which constitutes the invisible but living weapon of these inhabitants of the waters -that which, awakened by the contact of moist and dissimilar particles, circulates through all the organs of animals or plants-that which, flashing amid the roar of thunder, illuminates the wide canopy of heaven-which binds iron to iron, and directs the silent recurring course of the magnetic needle, -all, like the refracted rays of light, flow from one common source, and all blend together into one eternal all-pervading power."

The second group of the Malacopteri is that of the Abdominalia, in which there are ventral fins, and these are abdominal in position. Space will not permit of more here than merely mentioning that in this section are con tained amongst others the well-known and important groups of the Clupeide (Herring tribe), the Pikes (Esocida), the Carps, Barbels, Roach, Chub, Minnow, &c. (Cyprinida), and the Salmonida, comprising the various species of Salmon and Trout. Also belonging to this group are the Sheatfishes (Silurida), which are chiefly noticeable because they are amongst the small number of living fishes possessed of structures of the same nature as the fossil spines known as "ichthyodorulites." The structure in question consists of the first ray of the pectoral fins, which is largely developed, and constitutes a formidable spine, which the animal can erect and depress at pleasure. Unlike the old "ichthyodorulites," however, the spines of the Silurida have their bases modified for articulation with another bone, and they are not simply hollow and implanted in the flesh. The "Siluroids" are also remarkable for their resemblance to certain of the extinct Ganoid fishes (e.g., Pterichthys, Coccosteus, &c.), caused by the fact that the head is protected with an exoskeleton of dermal bones. The largest European species is the Silurus glanis of the Swiss lakes, and of various European rivers. Another remarkable member of this family is the Malapterurus of the Nile and west coast of Africa, which is endowed with electrical powers.

SUB-ORDER B. ANACANTHINI. This sub-order is distinguished by the fact that the fins are entirely supported by "soft" rays, and never possess "spiny" rays; whilst the ventral fins are either wanting, or, if present, are placed under the throat, beneath or in advance of the pectorals, and supported by the pectoral arch. The swim-bladder may be wanting, but when present it does not communicate with the oesophagus by a duct.

As in the preceding order, the Anacanthini are divided into two groups, distinguished by the presence or absence of the ventral fins. In the first of these groups (Apoda) are only a few fishes, of which one of the most familiar examples is the little Sand-eel (Ammodytes lancea), which occurs on all our coasts. In the second group (Sub-brachiata) in which ventral fins exist, are the two important families of the Gadida and Pleuronectida. The Gadida or Cod family, comprising the Haddock, Whiting, Ling, and Cod itself, is of great value to man, most of its members being largely consumed as food. In the Pleuronectida or Flat-fishes are comprised the Sole, Plaice, Turbot, Halibut, Brill, and others, in all of which there is a very curious modification in the form of the body. The body, namely, in all the Flat-fishes (fig. 243) is very much compressed

Fig. 243.-Pleuronectidæ. Rhombus punctatus. Natural size (after Gosse).

from side to side, and is bordered by long dorsal and anal fins. The bones of the head are twisted in such a manner that the two eyes are both brought to one side of the body, which is sometimes the right side, sometimes the left. The fish usually keeps this side uppermost, and is dark-coloured on this aspect; whilst the opposite side, on which it rests, is white. From this habit of the Flat-fishes of resting upon one flat surface, the sides are often looked upon as the dorsal and ventral surfaces

of the body. This, however, is erroneous, as they are shown by the position of the paired fins to be truly the lateral surfaces of the body. The mouth has its two sides unequal, the pectorals are rarely of the same size, the ventrals look like a continuation of the anal fin, and the branchiostegal rays are six in number.

SUB-ORDER C. ACANTHOPTERI.-This sub-order is characterised by the fact that one or more of the first rays in the fins are in the form of true, unjointed, inflexible, "spiny" rays The exoskeleton consists, as a rule, of ctenoid scales. The ventral fins are generally beneath or in advance of the pectorals, and the duct of the swim-bladder is invariably obliterated.

This sub-order comprises two families:

a. The Pharyngognathi, in which the inferior pharyngeal bones are an chylosed so as to form a single bone, which is usually armed with teeth. The family is not of much importance, the only familiar fishes belonging to it being the "Wrasses" (Cyclolabrida).

This

b. The Acanthopteri veri, characterised by having always spiny rays in the first dorsal fin, and usually in the first rays of the other fins, whilst the inferior pharyngeal bones are never anchylosed into a single mass. family includes many subordinate groups, and may be regarded as, on the whole, the most typical division of the Teleostean fishes. It will not be necessary, however, to do more than mention as amongst the more import ant fishes contained in it, the Perch family (Percida), the Mullets (Magilida), the Mackerel family (Scomberida), the Gurnards (Sclerogenida), the Gobies (Gobiida), the Blennies (Blenniida), and the Anglers (Lophiida). The Percida form by far the most important member of this group, and are distinguished by having ctenoid scales, the operculum and præ-operculum variously armed with spines, teeth on the vomer and palate as well as on the jaws, and the branchiostegal rays from five to seven in number.

SUB-ORDER D. PLECTOGNATHI.-This sub-order is characterised by the fact that the maxillary and premaxillary bones are immovably connected on each side of the jaw. The endoskeleton is only partially ossified, and the vertebral column often remains permanently cartilaginous. The exoskeleton is in the form of ganoid plates, scales, or spines. The ventral fins are generally wanting, and the air-bladder is destitute of a duct.

The most remarkable fishes of this section are the Trunk-fishes (Ostraci ontida, in which the body is entirely enclosed, with the exception of the tail, in an immovable case, composed of large ganoid plates, firmly united to one another at their edges.

Besides the Trunk-fishes, this section also includes the File-fishes (Balistide) and the Globe-fishes (Gymnodontida).

SUB-ORDER E. LOPHOBRANCHII.-This is a small and unimportant group, mainly characterised by the peculiar structure of the gills, which are arranged in little tufts upon the branchial

arches, instead of the comb-like plates of the typical bony fishes. The endoskeleton is only partially converted into bone, and the exoskeleton, by way of compensation, consists of ganoid plates. The swim-bladder is destitute of an air-duct.

The singular Sea-horses (Hippocampida), now kept in most of our large aquaria, belong to this sub-order, but the only point about them which requires notice is the curious fact that the males in this family are provided with a sort of marsupial pouch, into which the eggs are placed by the female, and to which the young, when hatched, can retire if threatened by any danger. This singular cavity is only found in the males, and is situated at the base of the tail. More familiar than the Sea-horses are the Pipe-fishes (Syngnathida), of which one species occurs commonly on our shores.

CHAPTER LV.

GANOIDEI.

ORDER IV. GANOIDEI.-The fourth order of fishes is the large and important one of the Ganoid fishes, represented, it is true, by few living forms, but having an enormous development in past geological epochs. For this reason the study of the Ganoid fishes is one which claims considerable attention.

At the present day, the order Ganoidei comprises only seven genera-viz., Lepidosteus, Polypterus, Calamoichthys, Amia, Acipenser, Scapirhynchus, and Spatularia- all of which are found only in the northern hemisphere, and are wholly or partially confined to fresh water.

The order Ganoidei may be defined by the following characters:-The endoskeleton is only partially ossified, the vertebral column mostly remaining cartilaginous throughout life, especially amongst the extinct forms of the Paleozoic period, in which the notochord is persistent. The skull is furnished with distinct cranial bones, and the lower jaw is present. The exoskeleton is in the form of ganoid scales, plates, or spines. There are usually two pairs of limbs, in the form of fins, each supported by fin-rays. The first rays of the fins are mostly in the form of strong spines. The pectoral arch has a clavicle, and the posterior limbs (ventral fins) are placed close to the anus. The caudal fin is mostly unsymmetrical or "heterocercal." The swim-bladder is always present, is often cellular, and is provided with an air-duct. The intestine is often furnished with a spiral valve. The gills and opercular apparatus are essentially the same as in the Bony fishes. The heart has one auricle and a ventricle, and the base of the