xiii PHYLUM CHORDATA 105 



always contractile, and contains several rows of valves. The 



> 



general course of the circulation is the same in all (see p. 145). 

 with some variation in the precise arrangement of the vessels. 

 In some of the Ravs the ventral aorta and the roots of the afferent 



t, 



vessels are partly enclosed in the cartilage of the basi-branchial 



-L / 



plate. 



The brain attains a much higher stage of development than 

 in the Cyclostomata. The fore-brain greatly exceeds the other 

 divisions in size. In Scymnus there are two widely-separated 

 parencephalic lobes or cerebral hemispheres containing large lateral 

 ventricles. In other genera there is at most, as in the Dog- 

 fish, a median depression of greater or less depth, indicating a 

 division into two lateral portions. In Scyllium, as already pointed 

 out, there is a median prosoccele which gives rise anteriorly to 

 two lateral ventricles, or paracoeles, and the same holds good of 

 Rhina and Acanthias. In the Rays there is only a very small 



i/ t^ t/ 



prosoccele without anterior prolongations. The olfactory lobes are 

 of great size, with in some cases short and thick, in others longer 

 and narrower, stalks. In Scyllium, Rhina, and Acanthias, as well 

 as in Scymnus, they contain ventricles : in the Ravs they are solid. 



i/ . t t/ 



The dieneephalon is of moderate extent. On its lower aspect are 

 a pair of rounded lobi inferior es, which are of thenature of a bilobed 

 dilated infundibulum, and a median saccus infundibuli and saccus 

 vasculosus, both diverticula of the infundibulum ; directly behind 

 the saccus vasculosus lies the hypophysis. The cpiphysis is long 

 and narrow. 



In the hind-brain the cerebellum is relatively greatly elongated, 

 and overlaps the optic lobes and sometimes also the dieneephalon 

 in front. Behind it extends over the anterior part of the medulla 

 oblonyata. It usually contains a cerebellar ventricle or cpiccelc. 

 The medulla is elongated in the Sharks, shorter and more triangular 

 in the Rays. The Electric Ravs are characterised by the 



/ t/ t/ 



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

 the fourth ventricle. 



Organs of Sense. --The sense-organs of the integument* &re 

 almost always simple or enclosed in branched canals, the mucous 

 or sensory canals, and are supplied by the lateral branch of the 

 vagus, and by branches of the trigeminal, facial, and glosso- 

 pharyngeal. On the head and anterior trunk region larger or 

 smaller canals occur having a number of dilatations the ampullce, 

 filled with gelatinous matter given off from them ; in these are 

 nerve-endings. Some Sharks are exceptional in the presence of 

 an open groove for the lodgment of the integumentary sense- 

 organs. 



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. 



