. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. OLFACTORY ORGAN. 171 These facts in the structure and development of the olfactory organ and nerve have caused an attempt to be made to draw a parallel between the olfactory pit and a primitive preoral gill-cleft,1 and this is further supported by the general structure and histological relations of the olfactory mucous membrane, which corresponds with that of the gills of Fishes in the possession of end-bulbs. In a recent paper, however, Beard has put forward the view that "the nose is real
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. OLFACTORY ORGAN. 171 These facts in the structure and development of the olfactory organ and nerve have caused an attempt to be made to draw a parallel between the olfactory pit and a primitive preoral gill-cleft,1 and this is further supported by the general structure and histological relations of the olfactory mucous membrane, which corresponds with that of the gills of Fishes in the possession of end-bulbs. In a recent paper, however, Beard has put forward the view that "the nose is really a branchial (segmental) sense-organ, the sense- organ of a non-existent gill-cleft, and not a gill-cleft ; Fishes.—In Petromyzon (Fig. 49, N, Net) and Myxin- oids the olfactory organ consists of a sac, unpaired externally, lying close in front of the cranial cavity, and opening on the dorsal surface of the anterior part of the head by a longer or shorter chimney-like tube. Its mode of development and internal structure, however, as well as the double olfactory nerve, seem to prove that the olfactory organ of Cyclostomes must also have been primitively a paired The position of the olfactory organ in Elasmobranchs differs from that of Cyclostomes in lying on the under instead of the. FIG. 141.—ANTERIOR PORTION OF HEAD OF Acipenser sturio. a, anterior, b, posterior opening of external nostrils ; o, isolated rosette of olfactory folds. upper surface of the snout. From these Fishes onwards throughout all Vertebrates the organ is always paired, and is more or less completely enclosed by a cartilaginous or bony investment, which forms an outwork of the skull. CFrom the Ganoid son wards it always has a similar position with regard to the skull, being situated between the eye and the end of the snout, either laterally or more or less dorsally. , In the course of development each external nostril of Ganoids and Teleostei becomes divided into two portio
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