Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 OLFAC1OBY ORGAN. 91 amongst at different distances, special apparatuses for its movement and accommodation are necessary. They are represented by muscles which can in the former case move the optic bulb and modify the direction of sight in obedience to the will of the animal, and in the latter act upon the rafractile media, and vary their relation to the retina. In many compound eyes (Decapod Crustacea) that part of the head on which the eye is placed i
Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 OLFAC1OBY ORGAN. 91 amongst at different distances, special apparatuses for its movement and accommodation are necessary. They are represented by muscles which can in the former case move the optic bulb and modify the direction of sight in obedience to the will of the animal, and in the latter act upon the rafractile media, and vary their relation to the retina. In many compound eyes (Decapod Crustacea) that part of the head on which the eye is placed is prolonged so as to give rise to a movable stalk-like process, which bears the eye at its extremity. The eyes of Vertebrata possess in addition special protective arrangements, , eyelids, lacrymal glands. The position and number of the eyes present very great variations the lower animals, paired arrangement on The the head appears to be the general rule among the higher animals; nevertheless visual organs sometimes occur on parts of the body far removed from the brain, as for instance, in Euphausia, Pecten, Spondy- lus, and certain Annelids (Sabellidre). In the Radiata the eyes are repeated at the periphery of the body in each radius. In the star fishes they lie at the extreme end of the ambulacra! furrow at the tip of the arms, in the Acaleplue as the marginal bodies on the edge of the umbrella. The sense of smell appears to be less widely distributed. Its func- tion is to test the quality of gaseous matters anJ. to produce in consciousness the special form of sensation known as ' Smell.' Thi^- sense in aquatic animals which breathe through gills cannot be sharply marked off from that of taste. The small pits, standing in connec- tion with nerves and provided with an epithelial lining of hair-bearing sense cells, are to be looked upon as the simplest form of olfactory organ (Medusa?, Heteropoda, Cephalopoda). Nevertheless scattered hair cells (Lnmellibranchiata) may also
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