XIII PHYLUM CHORDATA 179 



short and thick, in others longer and narrower, stalks. In Scyllium, 

 Rhina, and Acaiithias, as well as in Scymnus, they contain 

 ventricles ; in the Kays they are solid. 



The dicncephalon is of moderate extent. On its lower aspect 

 are a pair of" rounded lohi inferiores, which are of the nature of 

 dilatations of the infvmdilndiim., and a snccus vasculosics, which 

 is a diverticulum of the infundibulum ; directly below the 

 saccus vasculosus lies the hypophysis. The cpipliysis is long and 

 narrow. 



In the hind-brain the cerehcUum is relatively greatly elongated 

 and overlaps the optic lobes and sometimes also the dicncephalon 

 in front, while behind it extends over the anterior part (jf the 

 medulla ohlonyata. It usually contains a cerebellar ventricle or 

 epiccele. 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 {/leurmnasts, 

 p. 107) 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 continuous 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 continuous 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 ampidlary 

 canals, with neuromasts contained in terminal enlargements or 

 ampidlce: these, which are peculiar to the Elasmobranchs, are 

 most numerous about the snout region. Of similar essential 

 character are the vesicles of Savi 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. Tha eijc has a 

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

 wall of the orbit by means of a cartilaginous stalk. A fold of the 

 conjunctiva resembling the nictitating mcmhranc, or third eyelid 

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

 of the membranous vestibule, which is partly divided into two 

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

 canals with their ampulla?, and also the aqttcductus vestibuli or 

 cndoly rnphatic duct — which opens on the exterior on the dorsal 



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