. The elasmobranch fishes . THE ELASMOBRANCH FISHES 69 Since the extraliyoids and extrabranchials botli serve the same purpose the two types may be described together. These structures support the free mar- gins of the gill septa and hence run parallel with the deeper visceral arches to which the septa are attached. They may be present on the hyoid and on all the branchial arches except the last, as in Acanthias and in Raia erinacea {, fig. 74). In others, while the extrahyoid is lost dorsally it may persist ven- trally, making five inferior and only four superior cartilages (Heterodontus
. The elasmobranch fishes . THE ELASMOBRANCH FISHES 69 Since the extraliyoids and extrabranchials botli serve the same purpose the two types may be described together. These structures support the free mar- gins of the gill septa and hence run parallel with the deeper visceral arches to which the septa are attached. They may be present on the hyoid and on all the branchial arches except the last, as in Acanthias and in Raia erinacea {, fig. 74). In others, while the extrahyoid is lost dorsally it may persist ven- trally, making five inferior and only four superior cartilages (Heterodontus francisci). In still others, both segments of the extrahyoid arch may be absent, and yet a full complement of extrabranchial arches on the first four branchials may be present (Trygon). In a reduction of the number of extrabranchials the posterior cartilages are the first to be absent. A fourth ex- ^m^- trabranehial may be lacking ventrally, leaving three below and four above {Scyllium). While the extrahyal seg- ments present are normally small, the extrabranchials over the branchial arches may be rig. 75. Sagittal section through a developing verte- well developed. Occasionally ^-'^^' S^^U^^^um canimla. (From Schauinsland.) the tins of the dorsal and ven- chd., notochord; , elastica externa; , elastica interna ; ep., chordal epithelium; iz., inner zone; ms., tral segments of the anterior middle zone; oz., outer zone. arches overlap as in Hetero- dontus (fig. 71). In most forms, however, the dorsal and ventral segments fail to touch (Acanthias), and in many they are relatively insignificant structures (Raia erinacea, fig. 74). We have said above that the extravisceral arch is normally composed of a superior and an inferior segment. In a number of species an interesting con- dition is found in which lateral pieces, extraseptalia, are also added. These may be present as flattened bands of cartilage between the external clefts (Torpedo; Raja clavata) or they may be flatte
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