. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 172 COMPARATIVE ANATOMY. greatly, according to the width of the fold of skin which separates them. The olfactory organ of Polypterus is more highly developed than that of any other Fish. It is not a 'simple sac-like involution, but consists of six radially arranged compartments1 separated from one another by complicated septa, and lying round a central spindle. A transverse section has_ somewhat tin'appearance of a cut orange. A short and distinct oval sac lies against the oll'aetory organ towar
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 172 COMPARATIVE ANATOMY. greatly, according to the width of the fold of skin which separates them. The olfactory organ of Polypterus is more highly developed than that of any other Fish. It is not a 'simple sac-like involution, but consists of six radially arranged compartments1 separated from one another by complicated septa, and lying round a central spindle. A transverse section has_ somewhat tin'appearance of a cut orange. A short and distinct oval sac lies against the oll'aetory organ towards the middle line, and is entirely shut off from the rest of the apparatus ; it receives a special branch of the olfactory r FIG. 142.—ANTERIOR PORTION OF THE HEAD OF Polyptertix. A, eye ; AN, AN}, anterior and posterior openings of the external nostril: t, t, t, apertures of the sensory tubes. The mucous membrane of the nasal organ of Fishes is always raised up into a more or less complicated system of folds, which may have a transverse, radial, rosette-like, or longitudinal (in re- spect to the cranial axis) arrangement. The branches of the olfac- tory nerve are distributed on them, and they serve to increase the olfactory surface. Dipnoi and Amphibia.—The olfactory organ of Dipnoi and Pereniiibranchiata is always enclosed within a complete or perforated cartilaginous capsule lying without the cranium proper (Figs. 54, NK, 143, and N), and its mucous membrane is raised into folds like those of Fishes. In all the other Am- phibia it becomes included within the cranial skeleton, and lies directly in the longitudinal axis of the skull in front of the cranial cavity. In Amphibia, turbinals appear for the first time (Fig. 144, C, S, E]: they are processes of the cranial skeleton projecting into the nasal cavity, and thus giving rise to an extension of the olfactory surface. These structures, slight traces only of which are present in tailed Amphibians, attain to a ver
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