. The elasmobranch fishes . By a peculiar liypertropliy modifications may arise wliich, altliough essen- tially like the primitive shagreen denticles in strnctnre, greatly differ from them in form. Such hypertrophy may result in the production of a fin spine like that in Heterodontus (fig. 88) and the Spinacidae; a tooth like that in the sawfishes, Pristis (fig. 38) and Pristiophorus; a sting like that in the sting rays (fig. 42); or it may result in other variously modified structures, such for ex- ample as the branchial rakers in Cetorhmus (fig. 44) and Ellin 0 don. Fin Spine In some of the
. The elasmobranch fishes . By a peculiar liypertropliy modifications may arise wliich, altliough essen- tially like the primitive shagreen denticles in strnctnre, greatly differ from them in form. Such hypertrophy may result in the production of a fin spine like that in Heterodontus (fig. 88) and the Spinacidae; a tooth like that in the sawfishes, Pristis (fig. 38) and Pristiophorus; a sting like that in the sting rays (fig. 42); or it may result in other variously modified structures, such for ex- ample as the branchial rakers in Cetorhmus (fig. 44) and Ellin 0 don. Fin Spine In some of the types in which fin spines are present they are so rudimentary as to be but little larger than enlarged scales, as exemplified in Cenfroscymnus. In others, as in Heterodontus (fig. 88) and Acanthias,they are pronounced structures. In general, they are located just anterior to the dorsal fins, the posterior one (fig. 36) being longer than the anterior. For almost half its length the spine is buried in the integument. The buried part is designated as the root or base and the exposed portion the crown or spine proper. If such a structure be removed and more closely studied, its deeply imbedded base is seen to be triangular in shape. The spine contains a large central cavity which when in place fits over a cartilage of the fin skeleton. The walls of the spine are made of dentine which in the crown consists of a double layer. The more superficial layer is bounded anteriorly and laterally by a layer of enamel, but enamel does not extend over the posterior groove which fits close up against the basal cartilage of the fin skeleton. A more or less compact laj^er of pigment {pg., fig. 40) separates the enamel (e.) in front from the layer of dentine (d.). The development of such a fin spine is of considerable interest. Figure 37a represents a sagittal section of an early stage of Acanthias in which a mass of the epidermis (ep.) has sunk into the dermis, just in front of the dorsal fin. The
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