OLFACTORY OKGAX. 



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 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, e.g., 

 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 

 (Sabellidae). 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 

 Acalephce as the marginal 

 bodies 011 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 rnd to produce in 

 consciousness the special form of sensation known as " Smell." This 

 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 (Medusae, Hetercpoda, Cephalopoda). Nevertheless scattered 

 hair cells (Lamellibranchiata) may also have to do with the same 

 sensation. In the Arthropoda the cuticular appendages of the 



FIG. 88. Transverse section through the human 

 eye (after Arlt). C, cornea ; L, lens ; Jr, iris 

 with pupil ; Co, ciliary body ; Gl, vitreous 

 humour ; R, retina ; So sclerotic ; Ch, choroid. 

 Ml, macula lutea; Po, papilla optica; 2V'o, 

 optic nerve. 



