. Chordate morphology. Morphology (Animals); Chordata. lepidotrichia GYROPTYCHIUS B POLYPTERUS nDTrrmmi ^^mmnxmnnin;^^ rrmn RAJA LINTEA SQUALUS. 5 ceratotrichia Figure 8-44. Fin rays and radiols as observed in a choonate, on actinopterygian, and two chondrichthions. (After Jorvik, 1959) outward through the body cover, thus exposing the lepido- trichia as the outermost layer of scales. In the primitive actinopterygian fish, there is sometimes a gradual transition from body scale to lepidotrichium just as in the primitive choanate. The lepidotrichium is bony, jointed, branched, and guttered on i
. Chordate morphology. Morphology (Animals); Chordata. lepidotrichia GYROPTYCHIUS B POLYPTERUS nDTrrmmi ^^mmnxmnnin;^^ rrmn RAJA LINTEA SQUALUS. 5 ceratotrichia Figure 8-44. Fin rays and radiols as observed in a choonate, on actinopterygian, and two chondrichthions. (After Jorvik, 1959) outward through the body cover, thus exposing the lepido- trichia as the outermost layer of scales. In the primitive actinopterygian fish, there is sometimes a gradual transition from body scale to lepidotrichium just as in the primitive choanate. The lepidotrichium is bony, jointed, branched, and guttered on its inner surface. Its basal part into the body to overlap the radial. The num- ber of fin rays, or lepidotrichia, was usually greater than the number of radials, but in advanced forms these were generally equal. In advanced forms, such as the teleost, the base of the fin may be scaled and scales may even cover all or most of the fin. Where such scales they have no re- lationship with the underlying lepidotrichium. The lepidotrichia o{ Polyptems are of interest, because they have denticles and an ornamentation of ridges of dentine and enamel. Separate tooth-bearing plates may occur on the underside of the pectoral fins between the fin rays. The spines of the dorsal finlets of Polypterus are formed from modified lepidotrichia as are the spines of teleosts. The margins of the fins of teleosts and the adipose fin, where such occurs, are supported by unjointed horny rods which resemble ceratotrichia. These rods have been called actinotrichia because of their finer structure. In the grow- ing fin, one observes the margin supported by these rods which are replaced by the more superficial lepidotrichia with further growth. Embr/o/ogjcal development Ontogenetically, the various types of fin rays arise in the same way. They appear as thickenings of the basement membrane of the epidermis which are cut away by the movement of mesenchyme cells between this thickening and the
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