210 



COMPARATIVE ANATOMY 



danus, and the Holocephali, it is very narrow and elongated, while in 

 the rest of the Plagiostomi the individual parts are more closely 

 compressed and approximated together (Fig. 157). In almost all 

 Sharks the telencephalon is relatively much larger than any of the 

 other parts. The olfactory lobes arise from the anterior or antero- 

 lateral ends of the telencephalon, and in some Elasmobranchs 

 remain in close connection with it : in others, in which the 



olfactory capsules are situated further 

 forwards, they become drawn out into 

 long olfactory tracts, each arising from 

 a basal olfactory tubercle and continuous 

 anteriorly with an olfactory lulb from 

 which the olfactory nerves arise. 



A division of the telencephalon into 

 paired halves is hardly indicated at all 

 in Rays, and only slightly so in the 

 commoner Dogfishes (e.g. Scyllium, 

 Acanthias), in which, however, lateral 

 and olfactory ventricles are present 

 (Fig. 158). Only in Scymnus, and to 

 some extent in the Notidanidse, is there 

 a distinct separation of the pallium 

 into two hemispheres. In Rays there 

 is only a small unpaired teloccele, the 

 telencephalon consisting of a practic- 



a % s lid mass ' an . d th f ol &ctory lobes 

 (From Parker and are also solid ; in the MyhobatidaB 

 there is no trace at all of a telocoele. 

 The narrow diencephalon is roofed 



and the roofs of the various over by a choroid plexus, and the tube- 

 ventricles removed so as to 1-1 ' r / . m j \ 

 show the relations of the llke epiphysis (wanting in Torpedo) 

 cavities (semi-diagrammatic), may reach such a length as to extend 

 cer,. dilatation from which the Beyond the anterior end of the brain 

 metaccele is given off; dia, for a considerable distance, and pass 

 diaccele-the reference line distally into or beyond the roof of the 

 ^i^& h rt? <*: no indication can be seen of a 

 iter(mesoccele), into which the parietal organ. A pair of small lobes 

 optocoeles (opt) open; meta, the loli inferiores are present on 

 ?*',2lli * he infundibulum, and * saccns vascn- 

 of teloccele ; rh, rhinocrele. losus or infundibular gland, surrounded 



by a blood sinus, is present on the 



sides and floor of the infundibulum, with the ventricle in which it 

 communicates and with which a pituitary body is connected 

 posteriorly. 



The cerebellum is always very large, overlapping the* optic 

 lobes and medulla oblongata to a greater or less extent : it is 

 divided into several lobes lying one behind the other, and 

 usually contains a metacoele opening into the fourth ventricle. 



melct 



FIG. 158. BRAIN OF Cheilo- 

 scyllium. 

 Haswell's Zoology. ) 



Viewed from the dorsal side, 



