Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 OLFACIOET ORGAN. 91 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 refractile media, and vary their relation to the retina. In many compound eyes (Decapod Crustacea) that part of the iiead on which the eye i
Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 OLFACIOET ORGAN. 91 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 refractile media, and vary their relation to the retina. In many compound eyes (Decapod Crustacea) that part of the iiead on which the eye is placed is prolonged so as to give lise to a movable stalk-like process, which bears the eye at its extremity. The eyes of Vertebrata possess in addition special protective ari^xngements, , eyelids, lacrymal glands. The position and number of the eyes present very great variations amongst the lower animals. The paired arrangement on 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 (Sabellidje). 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 ambulacral furrow at the tip of the arms, in the Acalephre 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 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 j^its, 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?, Hetercpoda, Cephalopoda). Nevertheless scattered hair cells (Lamell
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