X 



SPECIAL SENSES 

 SPECIAL SENSES OF HEPTANCHUS MACULATUS 



Under the organs of special sense in Heptanchus we may consider the olfac- 

 torj^ organ, the eye, the ear, and the lateral line and associated organs. For the 

 relation of the first three of these to the brain see figure 200a. 



Olfactory Organ 



The olfactory organ in Heptanchus is an almost terminal encapsulated struc- 

 ture, with a convoluted mucous lining. From the outside this organ is reached 

 by the nasal aperture which is separated by an overlapping median flap into 

 an upper and a lower opening through which the water current circulates. 

 Upon dissecting away the narial flaps the cup appears elliptical in outline and 

 the lamellae are arranged in a series of folds which radiate outward from a 

 central hub, like the spokes in a wheel. 



Located in the convoluted lining of the olfactory membrane are the primary 

 olfactory cells (receptors). These cells send fine projections, hair cells, into 

 the cup and they also send fibers posteriorly to the olfactory bulb in two 

 bundles. These bundles compose the olfactory nerve, one part of which passes 

 to the olfactory organ externally and the other internally {I, fig. 200a) . Olfac- 

 tory sensations are carried backward by the olfactory tracts or peduncles 

 (ol.t.) to the olfactory lobes (ol.l.) of the brain. 



Eye 



The eye in Heptanchus is provided with dorsal and ventral membranous and 

 non-functional lids, but it is unprovided with a third eyelid or nictitating 

 membrane. It appears to fit the orbit but poorly, especially in the young in 

 which the orbit is unusually large. 



If the eyeball be dissected free from the orbit (figs. 91 and 200a) it is seen 

 to be slightly pear-shaped and to be held in position by the optic pedicel (o.p., 

 fig. 91) and the eye muscles which were described in a study of the muscular 

 system (pp. 89-90) . The coat which appears externally covering the eye is the 

 heavy sclera. In Heptanchus the sclera is chondrified and surrounds the ball, 

 except laterally over the clear elliptical cornea; in the center of the cornea is 

 the pupil surrounded by a dark colored curtain, the iris. 



Ear 



The ear proper (figs. 200a and 226) is located in the otic capsule. It is composed 

 of the semicircular canals with their ampullae and the utricular and saccular 

 regions. The anterior oblique and the horizontal canals have their anterior 



[258] 



