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TOSHIAKI J. HARA
and morays (Anguilliformes), with the most acute demonstrated sense
of smell, have large and elongate olfactory pits, extending from the tip
of the snout to the orbit of the eye. In contrast, certain puffers (Tetraodontidae ) which are highly visually oriented reef fishes, have completely
lost the nasal sacs. Various patterns of intermediate anatomical development can also be seen reflecting the relative role of olfaction in different
fishes. Each nasal pit generally has two openings which are separated
by an area of skin (Fig. 1). Olfactory currents of water enter the anterior
and leave through posterior openings-either passively through the
locomotion of the fish in the water, or actively by ciliary action within
the pits or by the action of muscles associated with the jaws or gills or
by some combinations of these methods (Burne, 1909; Pipping, 1926;
Teichmann, 1954). In the lungfish the external nostrils are true anterior
nares, whereas the internal nares open into the mouth in a manner corresponding to the choanae of higher vertebrates.
Lining the nasal sacs is the olfactory epithelium, which is generally
raised from the floor of the organ into a complicated series of folds to
make rosettelike arrangements ( Fig. 1). The olfactory folds vary greatly
in direction and number. Through these folds the total area of the sensory
epithelium is greatly increased. Burne ( 1909) distinguished oval (in most
fishes), round (in Esox) and elongate ( Anguillu) olfactory rosettes.
Species with elongate rosettes have the most numerous lamellae, which
are set at right angles to the longitudinal axis of the nasal sacs; such
rosettes can be generally correlated with an acute sense of smell (macrosmatic). Species with round rosettes, on the other hand, normally have
only a few lamellar folds and usually show no or minimal behavioral responses to olfactory stimulation (microsmatic). Species with oval rosettes
are most common and intermediate between the other two.
There have been several attempts to relate the total area of the olfactory epithelia in different species to their particular olfactory sensitivities.
Measurements of the surface area of the olfactory epithelia of eleven
species of freshwater teleosts (Teichmann, 1954) made it clear that
species with round rosettes had the smallest area of olfactory epithelium
(Esox, about 0.2% of the whole body surface; Gasterosteus, 0.4%)) and
that the broadest olfactory epithelium was not found in fishes with
elongate rosettes but in species with oval rosettes (Gobio, 3.6%; Phoxinus,
1.9%). In species with the elongate rosettes, the olfactory epithelium was
found to be 1.4% of the whole body surface in Anguillu and 1.3% in Lota
hta. However, there is no simple relation between the area of the olfactory epithelium and the number of receptors it contains. It is therefore
doubtful whether any simple relation exists between the area of the olfactory epithelium and sensitivity to odors.
TOSHIAKI J. HARA
and morays (Anguilliformes), with the most acute demonstrated sense
of smell, have large and elongate olfactory pits, extending from the tip
of the snout to the orbit of the eye. In contrast, certain puffers (Tetraodontidae ) which are highly visually oriented reef fishes, have completely
lost the nasal sacs. Various patterns of intermediate anatomical development can also be seen reflecting the relative role of olfaction in different
fishes. Each nasal pit generally has two openings which are separated
by an area of skin (Fig. 1). Olfactory currents of water enter the anterior
and leave through posterior openings-either passively through the
locomotion of the fish in the water, or actively by ciliary action within
the pits or by the action of muscles associated with the jaws or gills or
by some combinations of these methods (Burne, 1909; Pipping, 1926;
Teichmann, 1954). In the lungfish the external nostrils are true anterior
nares, whereas the internal nares open into the mouth in a manner corresponding to the choanae of higher vertebrates.
Lining the nasal sacs is the olfactory epithelium, which is generally
raised from the floor of the organ into a complicated series of folds to
make rosettelike arrangements ( Fig. 1). The olfactory folds vary greatly
in direction and number. Through these folds the total area of the sensory
epithelium is greatly increased. Burne ( 1909) distinguished oval (in most
fishes), round (in Esox) and elongate ( Anguillu) olfactory rosettes.
Species with elongate rosettes have the most numerous lamellae, which
are set at right angles to the longitudinal axis of the nasal sacs; such
rosettes can be generally correlated with an acute sense of smell (macrosmatic). Species with round rosettes, on the other hand, normally have
only a few lamellar folds and usually show no or minimal behavioral responses to olfactory stimulation (microsmatic). Species with oval rosettes
are most common and intermediate between the other two.
There have been several attempts to relate the total area of the olfactory epithelia in different species to their particular olfactory sensitivities.
Measurements of the surface area of the olfactory epithelia of eleven
species of freshwater teleosts (Teichmann, 1954) made it clear that
species with round rosettes had the smallest area of olfactory epithelium
(Esox, about 0.2% of the whole body surface; Gasterosteus, 0.4%)) and
that the broadest olfactory epithelium was not found in fishes with
elongate rosettes but in species with oval rosettes (Gobio, 3.6%; Phoxinus,
1.9%). In species with the elongate rosettes, the olfactory epithelium was
found to be 1.4% of the whole body surface in Anguillu and 1.3% in Lota
hta. However, there is no simple relation between the area of the olfactory epithelium and the number of receptors it contains. It is therefore
doubtful whether any simple relation exists between the area of the olfactory epithelium and sensitivity to odors.