174 ZOOLOGY SECT. 



epicosle. The medulla is elongated in the Sharks, shorter and more 

 triangular in the Rays. The Electric Rays are characterised by 

 the presence of the electric lobes, rounded elevations of the floor 

 of the fourth ventricle. 



Organs of Sense. Integumentary sense-organs (neuromasts, 

 p. 105) are highly developed in the Elasmobranchs. They are 

 supplied, as already mentioned, by branches of the nerves of what 

 is known as the lateral system, comprising, in addition to the lateralis, 

 nerves in relation with the facial and sometimes the glossopharyngeal. 

 These integumentary sense-organs occur in the interior of a con- 

 tinuous system of closed tubes, the sensory tubes, more rarely of open 

 grooves. The chief canals of this system are a lateral-line canal, 

 running along the middle of each side of the body, which is con- 

 tinuous with certain canals in the head : these communicate with 

 the exterior at intervals by small pores. In addition to the canals 

 of the lateral-line system there are a number of isolated canals, the 

 ampullary canals, with neuromasts contained in terminal enlarge- 

 ments or ampullce ; these, which are peculiar to the Elasmobranchs, 

 are most numerous about the snout region. Of similar essential 

 character are the vesicles of Sam which occur in the Electric Rays. 



The olfactory organs are a pair of cavities opening on the lower 

 surface of the head, a little distance in front of the mouth, and 

 enclosed by the cartilaginous olfactory capsules of the skull. Their 

 inner surface is raised up into a number of ridges on which the 

 fibres of the olfactory nerves are distributed. The eye has a 

 cartilaginous sclerotic, and is in most cases attached to the inner 

 wall of the orbit by means of a cartilaginous stalk. There appears 

 to be no mechanism providing for accommodation. A fold of the 

 conjunctiva resembling the nictitating membrane, or third eyelid 

 of higher Vertebrates, occurs in some Sharks. The ear consists 

 of the membranous vestibule, which is partly divided into two 

 (utriculus and sacculus), from which arise the three semicircular 

 canals with their ampullae, and also the aqueductus vestibuli or 

 endolymphatic duct which opens to the exterior on the dorsal 

 surface of the head. In the Rays the semicircular canals form 

 almost complete circles and open separately into the vestibule by 

 narrow ducts. 



Urinogenital Organs. The kidneys, as already noticed in 

 the account given of the Dog-fish, differ somewhat in their relations 

 in the two sexes. In the male the anterior portion persists in the 

 epididymis, and its duct becomes the spermiduct, while the posterior 

 portion, which is the functional kidney, has a duct or ducts the 

 ureter or ureters of its own. In the female there is no direct 

 connection between the reproductive and renal organs ; the anterior 

 portion of the kidney may be functional, and its duct persists, 

 opening along with those of the posterior portion. In the male 

 the ureters open into a median chamber the urinogenital sinus 



