The natural history of fishes, amphibians, & reptiles, or monocardian animals . pine-rayed groups,where, from the membrane being thin, the rays of thedorsals, ventrals, anals, and even the pectorals, may benumbered with accuracy. (44.) The external covering of the gills, and moreespecially their aperture, are of great importance, andrequire to be further noticed. The use of the gill-cover,or operculum, is obviously to protect the gills themselves,and, at the same time, to admit the egress of the watertaken in by the mouth: in the majority of typicalfishes it is moveable ; for, although compose
The natural history of fishes, amphibians, & reptiles, or monocardian animals . pine-rayed groups,where, from the membrane being thin, the rays of thedorsals, ventrals, anals, and even the pectorals, may benumbered with accuracy. (44.) The external covering of the gills, and moreespecially their aperture, are of great importance, andrequire to be further noticed. The use of the gill-cover,or operculum, is obviously to protect the gills themselves,and, at the same time, to admit the egress of the watertaken in by the mouth: in the majority of typicalfishes it is moveable ; for, although composed of bonyplates, these plates are articulated or jointed at theirsutures by a membranaceous skin which acts as a speaking, the operculum consists only of threepieces, which are attached to the cheek-bone, called thepre-operculum (Jig. 5. a)r of these three plates, the upperis more especially termed the operculum, and it is al-ways the largest; the next is the sub-operculum (6) ;and the third, which is very small and sub-triangular, 42 CLASSIFICATION OF is the inter-operculum (c), because it fills up the intervalbetween the base of the pre-operculum and the sno-oper culum. These distinctions will be rendered moreintelligible by the annexed figure, which represents the head of the common tench. These bones are eithersmooth both on their outersurface and their edges, orthey are armed, in one orboth situations, either withspines or prickles, or veryfine dentations like theteeth of a saw ; and these modifications are of greatimportance in determining generic groups. The hindermargin of the entire operculum, or gill-cover, is gene-rally bordered by a thin membranaceous skin, for thepurpose of closing the opening of the gills more ef-fectually : this skin is a continuation of that whichsupports the branchial rays, and these latter commenceat the bottom of the head, adjoining the throat; and- thenumber of these rays are considered indicative of ge-neric peculiar
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Keywords: ., bookcentury1800, booksubj, booksubjectfishes, booksubjectreptiles
