Segmentation is total but unequal in Acipenser and Lepidosteus, partial in the ova of all other Fish which are telolecithal. The embryo has a more or less prominent yolk-sac. Larval peculiarities are noticeable, e. g. the external gill-filaments of Elasmobranchii and some Teleostei; the external gills of Polypterus; the praeoral disc with suctorial papillae of Lepidosteus; peculiar spines, bony plates, growths on the fin in many Teleostei. Young Pleuro-nectidae have the eyes normal in position until they assume a horizontal posture in swimming, when one of the two rotates to the opposite side of the head. The Teleostean family Leptocephalidae appear to be arrested forms.

Fish, like Amphibians, are often sexually mature before they are mature in other respects. Their power of reproducing lost parts is confined to the fins. Many grow as long as life lasts, e. g. for over a hundred years (Carp, Pike); others attain a certain standard of size and are short-lived, e. g. Stickleback, Cyprinodonts, many Clupeids. Sexual dimorphism occurs, e.g. the male Teleostean is smaller than the female, and is often brilliantly coloured, temporarily or permanently. The flesh of some forms is poisonous always or at certain times, and is due in many cases to the food. The mucus of the body is often poisonous, and special poison glands are sometimes found, e.g. in connection with the dorsal spines of Synanceia (Scorpaenidae), or a perforated opercular spine, as in Thalassophryne (Batrachidae) among Teleostei Acanthopteri. The majority of fish are carnivorous; some omnivorous; a few, like the Mullets and Carps, vegetable-feeders.

Many fish are entirely marine, others exclusively fresh-water, while others again pass indifferently from fresh to sea-water, and vice versa, e.g. Pleuronectes. Some few ascend rivers to spawn, e. g. Acipenser, Salmo; others descend to the sea, e.g. Anguilla, the Eel. Instances are known of truly marine fish living in fresh water, e.g. a Shark in Lake Nicaragua; a Goby, Blenny, Atherina in the lakes of N. Italy. Marine fish are either littoral, pelagic, or abyssal. The latter are chiefly represented by Ana-canthini, a few Acanthopteri, and certain families of Physostomi. Eels descend to the greatest depths. Many Cyprinoidei and Muraenoidei of the temperate zones become quiescent in cold weather; and some, e.g. Carp, may be frozen without loss of life. Many tropical fish, e.g. some Siluroidei, Protopterus, burrow in mud during the hot season, when the rivers and pools dry up, and pass into a state of quiescence, which may last apparently for several years.

1 The male 15-spined Stickleback (Gasterosteus spinachia) spins a nest for the ova with a continuous thread. The material for the thread is secreted at the breeding season by the epithelium of the tubuli uriniferi and accumulates in the urinary bladder. Mobius, A. M. A. xxv. 1885; Prince, A. N. H. (5) xvi. 1885.

2 In two Australian Rays (Trygonorhina fasciata, Rhinobatis vincentianus) the shell contains more than one ovum - a unique peculiarity; Haacke, Z. A. viii. 1885. The same thing occurs occasionally in the common Hen.

The first remains of fish occur in Upper Silurian rocks, as spines, scales, cephalic shields, part of a jaw-bone, and coprolites, probably belonging for the most part to Elasmobranchii, some perhaps to Ganoidei. These two orders appear in numbers in Devonian strata, and some of the Ganoidei closely approach the Dipnoi, if they are not really Dipnoans. Ceratodus occurs in Permian strata, existing genera of Sharks in Cretaceous. Teleostei are found in the last-named rocks, and exceed the Ganoidei in numbers, but it is possible that the order is represented in the Lias. Teleostei replace Ganoidei almost completely in Tertiary times.

The Class Pisces may be subdivided into the following orders: I. Elasmobranchii(= Plagiostomt). Skeleton, for the most part cartilaginous; no investing bones; caudal fin heterocercal; paired fins large; ventral fins abdominal; copulatory organs present in male; mouth transverse and ventral; nostrils ventral; five to seven external gill-clefts, and usually a spiracle; upper jaw a moveable palato-quadrate cartilage; optic nerves forming a chiasma; a spiral valve in the intestine; a terminal cloaca and a contractile conus arteriosus with several rows of valves; ova large and few; impregnation internal; viviparous or oviparous; embryo with external gills. Contains two sub-orders: (I) Squalidae or Selachoidei, the Sharks, which are cylindrical in shape; have the gill-clefts placed laterally; free eyelids; and the pectoral limb not connected to skull; (2) Rajades or Batoidei, the Rays, which are flattened; have the gill-clefts ventrally placed; and the pectoral limb connected to the skull.

A Shark - Chlamydoselachus - from Japanese waters, has lately been described, which has pronged teeth like the Cladodonts of the Middle Devonian period. Its mouth is anterior as in other Vertebrata; its two nostrils are on the dorsal aspect of the face as in Teleostei; it has a large free opercular fold to the hyoid; six gill-clefts and an heterocercal tail scarcely bent up. It is the oldest living type of Vertebrata. See Garman, Bull. Mus. Comp. Zool. Harvard College, xii. No. 1, 1885.

II. Holocephali Differ From I

In having an opercular fold covering the gill-clefts, which are only four, and a naked skin; the palato-quadrate and hyoman-dibular coalesced with the skull; no cloaca but the anus, Miillerian and urinary ducts opening separately. Chimaera from the northern, and Callorhynchus from the southern hemisphere.

I. and II. are often grouped together as Chondropterygii.

III. Ganoidei

Skeleton, cartilaginous or ossified; investing bones always present in relation with cranium and shoulder-girdle; dermal skeleton forming large scutes or scales; ventral fins abdominal; an operculum with opercular bones; gill filaments free; a spiracle sometimes present as in I; optic nerves forming a chiasma; a spiral valve in the intestine; anus in front of a urogenital aperture; abdominal pores present; an air-bladder with pneumatic duct; a conus as in I; ova small, impregnated after exclusion.

The living Ganoidei are (1) Chondrostet, with a heterocercal caudal fin; naked skin, partially covered with large and small bony scutes and a notochord not divided into vertebrae. Acipenser, Scaphirhynchus, Spatularia (=Po/yodon). (2) Holostean, or bony Ganoidei, represented by three freshwater families, (i) Polypteridae with two genera, Polypterus and Calamoichthys, the former from West Africa and the Upper Nile, the latter from Old Calabar. The pectoral fins are lobate; the body covered with rhombic scales, the caudal fin homocercal. (ii) Lepidosteidae represented by Lepidosteus from North and Central America and Cuba, with fossil remains in Europe. The fins have fulcra; the caudal fin is slightly heterocercal; the scales rhomboid; the vertebrae opisthocoelous. (iii) Amiadae represented by Amia calva from North America. The scales are cycloid and large; the caudal fin homocercal.

There are many fossil forms. Accounts of them will be found in Gunther, Study of Fishes, 1880, and in the decades of the English Geological Survey.