. Un manuale di zoologia. Zoologia. IV. VERTEBRATA: pesci. 493 Sub Classe I. Myzontes {Hyperotrciia). Semiparasitic ciclostomi con cirri intorno alla bocca, molto primitivo nephridia, occhi rudimentali. Dalla grande quantità di muco sono noti come sUme anguille. Essi diedero in pesci e mangiare la carne. Myxine* costa orientale; Bdellostovia* west coast. Classe Siih. II. Petromyzontes (Hyperoartia). Il lamprede {Peiromyzm*) hanno ben sviluppato pinne dorsali, e sette gill aperture. Essi si verificano in sale e acqua fresca, alcune specie marine ascend-. Fig. 544.-Petromyzon marinus* lampreda di mare (dopo Goodc). ing s

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. A manual of zoology. Zoology. IV. VERTEBRATA: PISCES. 493 Sub Class I. Myzontes {Hyperotrciia). Semiparasitic cyclostomes with cirri around the mouth, very primitive nephridia, eyes rudimentary. From the large amount of mucus they are known as sUme eels. They bore into fishes and eat the flesh. Myxine* east coast; Bdellostovia* west coast. Siih Class. II. Petromyzontes (Hyperoartia). The lampreys {Peiromyzm*) have well-developed dorsal fins, and seven gill openings. They occur in salt and fresh water, some marine species ascend-. Fig. 544.—Petromyzon marinus* sea lamprey (after Goodc). ing streams to lay their eggs. The young have a larval (Ammoccetes) stage with rudimentary eyes and slit-like mouth. Many species live on mucus and blood which they rasp from fishes. Here may be mentioned a group of paleozoic fossils, the OSTRACO- DERMI, of uncertain position. They have fish-like bodies, but no skeleton or jaws are known. Ptcraspis, Cephalaspis, Pterichthys. Class II. Pisces (Fishes). The term fish is used in a wider and a narrower sense. In the first is includes any acjuatic vertebrate swimming by means of fins and breathing by gills; in the strict sense, as used here, it means aquatic branchiate forms with vertebral column, cranium, and well-developed visceral skeleton; with paired and unpaired fins, supported by a cartilaginous or bony skeleton; with double nasal pits; with a skin and oral mucous membrane which can produce ossifications, the scales and teeth. The fishes are the best adapted of all vertebrates for an aquatic life, and their whole organ- ization must therefore be considered from this standpoint. The epidermis consists of numerous layers of cells with an extremely thin cuticle. Cornifications of this epidermis are lacking under ordinary conditions, except a thin portion of the external subcuticular layer. At the time of sexual maturity cornifications increase greatly in most Cyprinoids and many Salmonids, producing hard bodies in the skin, the pea

Parole chiave associate all'immagine

• bookcentury1900
• bookdecade1910
• booksubjectzoology
• bookyear1912