In Amphioxus among the Acrania there is a ciliated pit above the anterior end of the central nervous system, which is probably a rudiment of an unpaired olfactory organ. In the Cyclostomata (lampreys and hags) the pit is at first ventral, but later becomes dorsal and shares a common open ing with the pituitary invagination. It furthermore becomes divided internally into two lateral halves. In fishes there are also two lateral pits, the nostrils of which open sometimes, as in the elasmobranchs (sharks and rays), on to the ventral surface of the snout, and sometimes, as in the higher fishes, on to the dorsal surface. Up to this stage the olfactory organs are mere pits, but in the Dipnoi (mud-fish) an opening is established from them into the front of the roof of the mouth, and so they serve as respira tory passages as well as organs for the sense of smell. In the higher Amphibia the nasal organ becomes included in the skull and respiratory and olfactory parts are distinguished. In this class, too, turbinal ingrowths are found, and the naso-lachrymal duct appears. In the lizards, among the Reptilia, the olfactory and respiratory parts are very distinct, the latter being lined only by stratified epithelium unconnected with the olfactory nerves. There is one true turbinal bone growing from the outer wall, and close to this is a large nasal gland. In crocodiles the hard palate is formed, and there is henceforward a considerable distance be tween the openings of the external and internal nares. In this order, too (Crocodilia) air sinuses are first found extending from the olfactory cavities into the skull-bones. The birds' arrange ment is very like that of the reptiles ; olfactory and respiratory chambers are present, and into the latter projects the true turbinal, though there is a pseudo-turbinal in the upper or olfactory cham ber. In mammals the olfactory chamber of the nose is variously developed; most of them are "macrosmatic," and have a large area of olfactory mucous membrane; some, like the seals, whale bone whales, monkeys and man are "microsmatic," while the toothed whales have the olfactory region practically suppressed in the adult, and are said to be "anosmatic." There are generally
five turbinal bones in macrosmatic mammals, so that man has a reduced number. The lowest of the series or "maxillo-turbinal" is the equivalent of the single true turbinal bone of birds and rep tiles, and in most mammals is a double scroll, one leaf turning up ward and the other down. Jacobson's organ first appears in am phibians, where it is found as an anteroposterior gutter in the floor of the nasal cavity. In reptiles the roof of the gutter closes in on each side, and a tube is formed lying below and internal to the nasal cavity, opening anteriorly into the mouth and ending by a blind extremity, posteriorly to which branches of the olfac tory and tri-geminal nerves are distributed. In the higher reptiles (crocodiles and chelonians) the organ is suppressed in the adult, and the same applies to birds; but in the lower mammals, espe cially the monotremes, it is very well developed, and is enclosed in a cartilaginous sheath, from which a turbinal process projects into its interior. In other mammals, with the exception of the Primates and perhaps the Chiroptera, the organ is quite distinct, though even in man, as has been shown, its presence can be demonstrated in the embryo.
See J. Symington on the organ of Jacobson in the Ornithorynchus, P. Zool. Soc. (1891), and in the kangaroo, J. Anat. and Phys., vol. 26 (1891) ; also G. Eliot Smith on Jacobson's organ, Anatom. Anzeiger, xi. Band No. 6 (1895). For general literature on the comparative anatomy of the olfactory system, see R. Wiedersheim's Comparative Anatomy of Vertebrates, translated and adapted by W. N. Parker (London, 1907). (F. G. P.)