CHAPTER VIII.

PISCES: ORDERS.

NATURALISTS have always experienced great difficulties in the classification of fishes, hence the variety of those proposed, and the confusion which has, in consequence, arisen. Subjoined are those now principally employed :—

1. Dipnoi (díπvool, double breathing).

2. Elasmobranchii (ěλaσμa, a thin plate; and Bpáyxiov). 3. Ganoidei (yavós, lustre ; and eîdos).

4. Teleostei (Téλeov, perfect; and ỏσtéov, a bone).

5. Marsipo-branchii (μápσiños, a pouch; and Spáɣxiov). 6. Pharyngobranchii (papvy§, the throat; and Spáɣxiov).

1. Selachia (σéλas, brightness).

2. Ganoidei.

3. Teleostei.

(Huxley.)

4. Cyclostomata (kúкλos, a circle; and σróμa, mouth).
5. Leptocardia (λéñw, I strip off; and κapdíα, the heart).

(Müller.)

1. Chondropterygii (xóvôpos, cartilage; and πeρúуiov).
2. Malacopterygii (μaλañóv, soft; and πτepúуiov).
3. Acanthopterygii (ǎkavēα, a spine; and πтepúуiov).

(Cuvier.)

1. Placoidei

2. Ganoidei.

3. Ctenoidei.

4. Cycloidei.

(Agassiz.)

It is intended, in the following pages, to adopt the classification proposed by Professor Huxley.

I. DIPNOI. This order is believed to form the connecting link between the fishes and amphibians, and only includes that remarkable fish called the mud-fish or lepidosiren. For a long time this animal was placed among the amphibians, but is now, by almost universal consent, ranked with the fishes. The mud-fishes inhabit the marshy districts of Brazil and the east coast of Africa. During the rainy season they live as fishes in the swollen rivers, but when the dry season approaches they burrow into the mud, which is soon hardened by the sun—an aperture is, however, left for the admission of air. In consequence of this peculiarity in their mode of life, they possess some of the characteristics common to both fishes and amphibians. The body is shaped like that of a fish, and is covered with small, horny, cycloid scales; both the pectoral and ventral fins are present, but they have the appearance of awl-shaped filaments; the posterior part of the body, both above and below, has a median fin. This order of animals, as the name imports, breathes both by gills and lungs. The gills are situated in a branchial chamber, which opens externally by a vertical slit; the lungs are in the form of a double cellular airbladder, which communicates with the esophagus by means of the ductus pneumaticus. In the young there are external gills, which soon disappear. The nasal sacs open posteriorly into the pharynx; this also occurs in the myxines; in all other fishes they end in cul de sacs behind. The heart consists of two auricles and a single ventricle. The skull is composed of separate bones, and the noto-chord is persistent. There are only two species known, viz. :—

a. Lepidosiren paradoxa, from the Amazon.
b. Lepidosiren annectens, from the Gambia.

II. ELASMOBRANCHII, also called selachia (Müller), placoidei (Agassiz), include all the sharks, dog-fishes, rays, and chimære. The members of this order possess the following characters:-The endoskeleton, as a rule, is cartilaginous. (in the plagiostomi the vertebral centra are usually more or less ossified); the skull forms a cartilaginous box. The heart consists of an auricle and ventricle, and the bulbus arteriosus, which has several rows of transverse valves, is provided with a distinct layer of muscular fibres, and contracts rhythmically. The branchiæ are fixed and pouchlike, have no operculum or branchiostegal rays, and generally open externally by five vertical slits (in the holocephali, however, there is a single gill slit, and the operculum and branchiostegal rays are rudimentary). The intestine is short, and has a fold of mucous membrane passing in a spiral manner from the pyloric extremity of the stomach to the anus. The integument has, irregularly scattered through it, detached grains or tubercles of bony matter. Both the pectoral and ventral fins are present, the latter being placed near the anus. The teeth in this order are very variable in their form, but there is a striking peculiarity in those of the sharks; in these there are several rows of teeth placed one behind the other, the front row composed of triangular cutting teeth, and when these are worn out, the succeeding row take their place, and so on. This order is sub-divided into

(A.) Holocephali (öλn, whole; and кɛpɑλń, head).—The only existing representatives of this sub-order are the chimærida; they are known by the following characters: The chorda dorsalis is persistent, but the transverse processes and neural arches are cartilaginous; the gills open by a single aperture externally, and the mouth is placed at the extremity of the head. They include

a. Chimæra monstrosa, or King of the Herrings.
b. Callorhyncus australis, or Southern Chimæra.

(B.) Plagiostomi (πλáуiov, transverse; and σróμa, mouth), containing the well-known shark and ray. In this suborder the vertebral column is more or less ossified; the mouth is placed on the under surface of the head, and is transverse; the branchiæ open externally by five vertical gill slits; two tubes pass from the pharynx, and open on the upper part of the head by two openings named "spiracles." They are divided into—

a. Cestraphori (Kéσтρа, a pointed weapon; and pepw, I
bear)-e. g., Port Jackson Shark.

B. Selachi-e.g., Sharks and Dog-fishes.
7. Batides-e. g., Rays.

III. GANOIDEI.-These were very numerous in former geological periods, but are now represented by only a few existing forms. They are all covered with ganoid* scales, plates, or spines, which are composed of a superficial layer of ganoine, a substance somewhat similar to enamel, resting on a layer of bone. The endoskeleton is generally cartilaginous, but sometimes more or less ossified, and in the bony pike (lepidosteus) the vertebræ are more completely ossified than in any other existing fish, while the vertebral centra are opisthocælous. The bulbus arteriosus contracts rhythmically, and has several transverse rows of valves; the intestine frequently has a spiral valve. The caudal fin is usually "hetero-cercal;" the gills are free, and covered by an operculum. The swimming bladder is always present, often divided by septa into several cells, and opens into the gullet by the ductus pneumaticus. Both pectoral and ventral fins are usually present, the latter placed far back. This order is sub-divided by Professor Owen into—

* The plecto-gnathi and lopho-branchii among the teleostei also have ganoid scales and plates.

a. Lepido-ganoidei (λenís, a scale; yávos and eldos)— e.g., Bony Pike (lepidosteus) and Polypterus.

b. Placo-ganoidei (#λág, a flat surface; yávos and eldos), e.g., Sturgeon.

IV. TELEOSTEI. This order comprises the majority of existing fishes, and is characterized by the possession of a more perfectly ossified endoskeleton than the other orders (bony pike excepted). A teleostian fish having been already taken as the type in the general description of the class pisces, it is not necessary to recapitulate the leading characters of this order; its sub-divisions are

* The first rays in the dorsal and pectoral fins are sometimes spiny.