FISH. In the popular sense a fish is a cold blooded, chordate or vertebrate animal, adapted for life in the water, breathing by if of gill slits all its life, having its limbs, f present, de veloped as fins, never as fingers or toes. This definition excludes invertebrates as having no notochord and no backbone. It excludes the tunicates and enteropneustans as not fish-like in form when adult and as being at all stages without fins. It excludes the amphibians, be cause these develop, in the adult state, jointed limbs with toes and most of them cease to breathe with gills or gill-slits. The fishes, as thus defined, include all members of the groups known as the lancelets (Leptocardii), the lam preys (Cyclostomi), the sharks (Davao bronchi°, and the true fishes (Teleostomi), as also all of the varied series of extinct fish-like forms. In technical writings the lancelets and the lampreys are usually excluded from the class Pisces or fishes and many writers would still further limit the name of Pisces by the exclusion of sharks and skates and also the chimwras (Holocephali). Still others exclude the Dipneusta and the extinct Placodermi or Ostracophori and Arthrodira. It is perhaps not necessary to regard Pisces or fishes as a technical term at all. In this case there is no violence in using the word fish as coextensive with the popular definition as given above. Taking this significance, we may recognize two classes of chordates (Tunicata and Enterop neusta) as lying below and to one side of the series of fishes, while this series is itself composed of nine classes or sub-classes unequal in size and in taxonomic value, Leptocardii, Cyclostomi, Ostracophori, Cyclic, Elasmo branchii, Arthrodira, Crossopterygii, Dipneusta, (Dipnoi) or (Plagiostomia) and Actinopteri. The first three groups differ widely from the others and must in any scheme of classification he regarded as forming distinct classes. The last-named class or sub-class includes the great majority of recent fishes. The Cyclic, Os tracophori and Arthrodira are wholly extinct; the Crossopterygii and Dipneusta nearly so.
Taking the true fishes, or Actinopteri, as typ ical of the group, we see at once that these have .the general structure of the higher verte brates, but with less complexity of structure, with various adaptations that fit these animals for life in the water.
Form of the body is in gen eral boat-shaped, the head in the same axis, without neck in most cases, the shoulder-girdle being attached to the skull at the nape and in the more specified forms of the pelvic girdle joined to the shoulder-girdle below. The form is fitted for swift progress through the water. The body is longer than deep and the greatest i width is in front of the middle, leaving in most cases the compressed paddle-like tad to serve as the organ of locomotion. To all of these statements there are numerous excep tions. Most fishes depend on speed to secure their food or to escape from their enemies, but there are some which preserve themselves by lying prone on the bottom, by hiding in crevices of one sort or another, or which are defended from all attacks by pungent, sometimes en venomed, spines or by a bony coat of mail.
Exoskeleton of the The surface of the fish is typically covered by an exoskeleton of overlapping scales. To this there are many exceptions. Some are naked, some covered with prickles, spines or bony plates. The scales may he ganoid (enameled, like teeth), placoid (reduced to shagreen-like roughness), ctenoid (with a comb-edge), or cycloid (smooth, with concentric striae)), or they may be variously coalescent, forming bony plates. Historically the placoid scale precedes the ganoid scale. This is followed by the cycloid, and finally by the ctenoid type. Bony plates may be formed by the coalescence of scales of any tYPe.
Fins of the Fish.— In most cases the motion of a fish is mainly produced by the lateral movement of a caudal or tail fin, the other fins exerting chiefly the function of direction. In the more primitive of existing fishes the dif ferent fins are composed of soft rays con nected by membrane. The soft rays are finely jointed and usually branched, as distinguished from the spines, which are without branches or joint and which are usually stiff and pungent. In the more specialized fishes spines are usually present, these occupying the front of the dorsal and anal fins, the first ray of the ventral fin being also spinous.