126 ELASMOBRANCHII 



85 and 196, where the nerve-supply of these canals is given). The 

 very characteristic groups of sensory ampullae, supplied by the 

 facial nerve (Fig. 84), are distributed on the head, as shown in 

 Figs. 1 1 and 85. These deep-seated organs communicate with the 

 exterior by long tubes opening by conspicuous pores on the surface 

 (Fig. 84). 



Among the characteristic features of the brain one may mention 

 the following. The prosencephalon (Fig. 7) is large, of paired 

 origin, and may contain paired cavities, but shows little or no sign 

 of external division, especially in the Eajiformes. The thickened 

 floor and wall of the telencephalon merges in front with that of the 



trf. gl. 



cba. 



Fio. 85. 



Diagram of the head of Laemargus, showing the cranial nerves and sense-organs. (After.Ewart 

 and Mitchell.) The lateral-line canals are indicated as in Fig. 196, p. 222. a, auditory labyrinth ; 

 ad, auditory nerve ; b, buccal branch of facial ; c, ciliary ganglion on branch of oculomotor ; 

 e, eye ; g, epibranchial ganglion on branch of vagus ; gl, glossopharyngeal ; li.a, hyoid group 

 of ampullae of Lorenzini ; hm, hyomandibular branch of facial ; i.ba, inferior group of ampullae ; 

 i.v, intestinal branch of vagus ; 1,1, lateral-line canal of trunk ; l.v, lateral-line branch of vagus ; 

 m.a, mandibular group of ampullae ; mx, maxillary branch of trigeminal ; n.c, nasal sac ; ob.a, 

 suborbital group of ampullae ; p.o, pit-organ ; pr, profundus ; p.tr, post-trematic branch of 

 branchial nerve ; .o, superior ophthalmic branch of trigeminus ; s.o.a, superior group of 

 ampullae ; s.o.f, superior ophthalmic branch of facial ; sp, spiracle ; st.v, supratemporal branch 

 of vagus ; t.r.f, roots of trigeminus and facial ; v, vagus root ; I-V, five branchial slits. 



prosencephalon, obliterating the thin lamina terminalis, and forming 

 a mass of nervous tissue covering the sides, front wall, and roof of 

 the telencephalon (Figs. 7, 86). It is into this often immensely 

 developed prosencephalic mass that the lateral ventricles project. 

 At the sides, or in front, arise large diverging olfactory lobes ; these 

 may either be near the cerebrum or they may be drawn out into 

 long olfactory tracts, expanding anteriorly into olfactory bulbs close 

 to the nasal capsules. The epiphysis is long, and reaches forwards 

 in Selachians to below the superior fontanelle of the skull; but 

 there is no pineal eye. As in many other fish, the lobi inferiores and 

 saccus vasculosus are well developed in the infundibular region. 

 The cerebellum is very large and often convoluted ; the restiform 

 bodies, at the sides of the medulla, may also be prominent. 



