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PROTOZOA AND CELENTERATA.

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the higher animals, we cannot wonder that in the lower groups our knowledge is still less.

In the Protozoa and Coelenterata no organs have yet been met with to which this function can with any confidence be ascribed.

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Meyer has described,* in Polyophthalmus (a small marine worm), on each side of the head, two ciliated organs (Fig. 33), which have been supposed to be organs. of smell. These had been already mentioned by *“Zur. Anat. und Hist. von Polyophthalmus,” Arch. für Mic. Anat., 1882.

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WORMS-MOLLUSCA.

De Quatrefages, who compared them with the ciliated wheels of Rotifers, and thought that they produced currents in the water, thus urging microscopic algæ, infusoria, etc., to the mouth of the worm. Meyer, on the contrary, with more probability, regards them as olfactory organs. They are slight depressions (Fig. 33) in the general surface, lined with peculiar long ciliæ, supplied with a large nerve coming from

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Fig. 33.-Section through the head segment of Polyophthalmus, × 300 (after Meyer). Imd, muscle; bo, cup-shaped organ; cu, cuticle; hp, hypoderm; Imd, longitudinal dorsal muscle: n, peripheral nerve; cz, commissure of brain; mb, membrane; pgn, pigment-cells; hpdz, unicellular glands in the hypoderm ; gn, brain ; k, nuclei in the brain.

the cerebral ganglion gn. Similar pits occur in many other Annelida. They differ in number; Polyophthalmus having only a pair, the Capitellidæ several.

In the Mollusca, the hinder pair of tentacles have been supposed by some to serve as olfactory organs, In the cuttle-fish (Cephalopoda) there are certain pits, at the base of which is a papilla, supplied with a nerve, which is perhaps olfactory.* The true function of the

* Leydig, "Histologie."

INSECTS-SEAT OF THE SENSE OF SMELL. 35

organs described by Hancock in Gasteropods, and by Leuckart in Pteropods, as olfactory, seems very doubtful.

As regards the seat of the sense of smell in insects, there have been four principal theories. It has been supposed to reside—(1) In the spiracles, or breathing holes; (2) in the neighbourhood of the mouth; (3) in the antennæ; (4) in different parts of the body. The history of the question has been well given by Kraepelin in an admirable memoir, "Ueber die Geruchsorgane der Gliederthiere." *

Sulzer, in 1761,† suggested that the organ of smell was probably to be found in the neighbourhood of the spiracles, or breathing-holes. It is hardly necessary to observe that insects do not breath as we do, through their mouths, but through a series of orifices along the sides, leading into trachea, or air-tubes, which ramify throughout the body; so that the blood is aerated, not in one special organ, but throughout its course. Now, it is important that a more or less continuous current of air should pass over the surface of the organ of smell, as it is in this manner brought in contact with the odoriferous particles. In man and the other airbreathing vertebrates, the combination of the entrance to the lungs with the nose and mouth offers great advantages. The olfactory organ is brought close to the mouth, where it is especially useful in the examination of food; while the continuous current of air necessary to respiration is utilized in the production of sound, on the one hand, and in bringing odoriferous particles to the organ of smell, on the other.

* Separat Abdruck aus dem Osterprogramm der Realschule des Johanneum." 1883.

t "Geschichte der Insekten."

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DIFFERENT THEORIES AS TO

In insects the separation of the mouth from the respiratory orifices is, in this respect, a manifest disadvantage. Still, it was not unnatural to look for the organ of smell in the neighbourhood of the spiracles, Sulzer's view was supported by Von Reimarus, Baster, Dumeril, Schelvir, and especially by Lehmann,* who lays it down as a general proposition that every organ of smell is to be sought near the orifices through which animals breathe: "Omnibus olfactus organon in iis locis quærendum est, per quos inspirent."

The most careful observations, however, have failed to detect in the neighbourhood of the spiracles any special supply of nerves, or any organ which could be supposed to serve for the perception of odors, and I believe this view may be said to be now generally abandoned.†

Treviranus suggested that the organ of smell was situated in the mouth, and he has been followed by Newport, Wolff, Kirby and Spence, and Graber. The descriptions they have given may be accepted as correct, but the organs they describe in the mouth itself are rather, I think, to be ascribed to the sense of taste than to that of smell.

Lyonnet, Bonsdorff, Marcel de Serres, Newport, and others, believed that the sense of smell resides in the

* Lehmann published three memoirs on the subject: "De Sensibus Externis Animalium Exsanguium," 1798; "De Antennis Insectorum Dissertatio," 1799; and "De Antennis Insectorum Dissertatio Posterior," 1800.

† Joseph, indeed ("Bericht der 50 Vers. Deutscher Nat. und Aerzte. München," 1877), supported this view in a short communication, and has promised fuller details. These, however, have not, I believe, yet appeared.

"Ueber das Saugen und das Geruchsorgan der Insekten," Ann. der Wetter Ges., 1812.

THE SEAT OF THE SENSE OF SMELL.

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palpi, although the experiments of Perris, Plateau, Forel, and others, have conclusively proved that it is not situated exclusively in them.

The credit has been ascribed to Réaumur of having been the first to suggest that the sense of smell is seated in the antennæ. This view has been adopted by Lesser, Roesel, Lyonnet, Bonnet, Sulzer, Latreille, Burmeister, Lefevre, Erichson, Dugès, Perris, Dufour, Slater, Vogt, Forel, Lowne, Hauser, Kraepelin, Schiemenz, and other observers, and my own observations lead me to the same conclusion.

Many entomologists, indeed, including Scarpa, Schneider, Bolkhausen, Bonsdorff, Carus, Strauss-Durckheim, Oken, Kirby and Spence, Newport, Landois, Hicks, Wolff, and Graber, have considered that the antennæ serve as ears. These two views are, however, not irreconcileable, and the truth seems to be that, while organs of smell and of hearing, when present, may be both situated in the antennæ, they are not in all cases confined to them.

Comparetti* seems to have been the first to suggest that the organ of smell might not be seated in the same part of the body in all insects; he suggested the antennæ in certain beetles (Lamellicornia), the proboscis in butterflies and moths (Lepidoptera), and certain frontal cellules (the existence of which has, however, not been confirmed) in locusts, etc. (Orthoptera), as the probable seats.

The real manner in which odors are perceived, and the structure of the olfactory organs, is still so little understood, that experiments are perhaps more conclusive than anatomy.

"De aure interna comparata-Patavii." 1789.

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