der of some species yields isinglass. The minute laminæ which give brilliancy of color to some, and the similar substance found in the air-bladder of others, afford the materials of which artificial pearls are made. Oil useful for lamps, etc., is obtained from several species, and the medicinal value of cod-liver oil is now well known. See Fisheries.
Classification. History of Ichthyology.—Among the ancient students of ichthyology, that branch of natural history which treats of fishes, the first to be mentioned, as usual, is Aristotle. In modern times ichthyology began to be cultivated about the middle of the sixteenth century by Belon, Rondelet, and Salviani. Their work was of value locally only. The first work of real value, and which marks the beginning of a system based on scientific principles, was that of Willughby and Ray, which appeared in 1686 under the title Historia Piscium. Here a distinct effort at classification was made. They divided all fishes into two classes, Cartilaginei and Ossei. Each of these classes was divided into two groups, on the basis of the form of the body—the Cartilaginei into Longi, including the sharks, and Lati, including the skates; and the Ossei into Plani, including the flatfishes, and Non-plani, including all others. It is at once evident how artificial this classification is. Artedi, whose writings, on account of his death, were published by Linnæus, worked out a system of classification considerably influenced by Willughby and Ray. He included the cetaceans among the fishes. His system was adopted by Linnæus in his earlier editions of Systema Natura, but later (1758) Linnæus devised an original classification, which, among other changes, eliminated the Cetacea from the fishes and placed them with the mammals. The classification worked out by Bloch and Schneider was superficial in the extreme. The number of fins present was the basis of their division into Monopterygia, Dipterygia, etc. This work was published in 1801. Bloch, in 1782-95, published a large and important work on fishes, comprising nine volumes with fine illustrated plates, in which he described about four hundred species. Several other authors wrote extensively on fishes during the last half of the eighteenth century and the beginning of the nineteenth. Among these is to be mentioned Lacépède, Histoire naturelle des poissons (5 vols., Paris, 1803), in which 1400 species were described. During the first quarter of the nineteenth century Cuvier did much on the classification of fishes, his system appearing in his Regne Animal (Bonn, 1830). The anatomist Johannes Müller published in 1846 a natural classification which influenced the systems to a very high degree. He divided fishes into Leptocardii, Marsipobranchii, Elasmobranchii, Ganoidea, Teleostei, and Dipnoi. Louis Agassiz (q.v.) advanced our knowledge both of living and fossil fishes. Influenced by the latter, he divided the class into four groups on the character of their scales: placoid, ganoid, cycloid, and ctenoid. This classification, though convenient in many ways for the study of fossil remains, was adopted by scarcely any of the authorities. Albert Günther, in his Catalogue of Fishes in . . . the British Museum (London, 1859-70), has largely modeled the modern system of classification. Among the recent more influential American ichthyologists are Theodore Gill, the late E. D. Cope, and David Starr Jordan, whose historical review of ichthyology in the Proceedings of the American Association for the Advancement of Science for 1902 is very complete.
Present Arrangement. The subphylum Vertebrata includes as the lowest in rank of its groups several series of fish-like vertebral divisible first into Acraniata (the lancelets [Leptocardii] only; see Amphioxus), and Craniata, which includes all the remainder. The fish-like Craniata fall into two classes:
I. Cyclostomata. Characterized chiefly by having ‘a suctorial mouth devoid of functional jaws,’ and by the absence of paired fins; these are the lampreys and hagfishes (orders Petromyzontes and Myxinoidei).
II. Pisces. Characterized by having the organs of respiration (gills) and the organs of locomotion (paired fins) adapted for an aquatic life. The class is divided into subclasses, as follows:
(1) Elasmobranchii.—Pisces with a skeleton composed essentially of cartilage—the sharks, rays, etc., divided into three orders, Cladoselachea, Pleuracanthea, Acanthodea, and Selachii. The first three are represented by Paleozoic fossil forms. The last includes many extinct and all the existing forms.
(2) Holocephali.—Shark-like Pisces, with a large compressed head and a single external branchial aperture. It includes only the family Chimæridæ (chimæras).
(3) Teleostomi.—Pisces ‘distinguished from the Elasmobranchii and Holocephali by having the primary skull and shoulder-girdle complicated by the addition of membrane bones, and by possessing bony instead of horn-like fin rays.’ This includes all of the common ‘bony fishes,’ as well as the so-called ganoid fishes. Its orders are: Crossopterygii (bichir, etc.); Chondrostei (sturgeons); Holostei (gar-pikes, etc.); Teleostei (bony fishes generally). The first three orders are frequently grouped together as ‘Ganoidei.’
(4) Dipnoi.—Pisces with lung-like respiratory organs as well as gills, and the fins constructed on the type of the archipterygium. It includes the lung-fishes, and by some authors is made a separate class altogether. Its orders are Monopneumona and Dipneumona.
(5) Ostracodermi (q.v.).—A group of uncertain limits and affinities, known only from Paleozoic fossils, “characterized by the extraordinary development of the exoskeleton (bony plates) of the head, and the absence, in all the fossil remains hitherto found, of endoskeleton, including jaws.”
Bibliography. Bloch, Allgemeine Naturgeschichte der Fische (Berlin, 1782-95); Cuvier and Valenciennes, Histoire naturelle des poissons (22 vols., Paris, 1828-49); Günther, Catalogue of Fishes in British Museum (London, 1859-70); Günther, Introduction to the Study of Fishes (Edinburgh, 1880); Dean, Fishes, Living and Fossil (New York, 1895). Consult also various works on the comparative anatomy of vertebrates, such as Huxley, Gegenbaur, Owen, Parker and Haswell, Wiedersheim, etc. Of faunal works the principal are: Jordan and Evermann, Fishes of North and Middle America (4 vols., Washington, 1896-1900); Goode and Bean, Oceanic Ichthyology (Washington, 1895); Goode, American Fishes (New York, 1888); annual Reports and Bulletins of the United States Commission of Fish and Fish-
