372 



COLLEGE ZOOLOGY 



brane well supplied with capillaries and 

 borne by the hyoid arch and first 4 branchial 

 arches. They are supported by these arches 

 and by gill rays. Water entering the mouth 

 passes between the branchial arches and out 

 through the gill slits and spiracles, thus 

 bathing the gills and supplying oxygen to 

 the branchial blood vessels. 



Nervous system 



The brain (Figs. 248 and 249) is more 

 highly developed than that of the cyclo- 

 stomes. It possesses two remarkably large 

 olfactory lobes, a cerebrum of two hemi- 

 spheres, a pair of optic lobes, and a cere- 

 bellum which projects backward over the 



medulla oblongata. There are 11 pairs of 

 cranial nerves if the terminal is included. 

 The nerve (spinal) cord is a dorsoventrally 

 flattened tube with a narrow central canal; 

 it is protected by the vertebral column. 

 Spinal nerves arise from its sides in pairs. 



Sense organs 



The two olfactory sacs are characteristi- 

 cally large in elasmobranchs. The ears are 

 membranous sacs, each with three semicir- 

 cular canals; they lie within the auditory 

 capsules. The eyes are well developed. Along 

 each side of the head and body is a longi- 

 tudinal groove called the lateral line; it 

 contains a canal with numerous openings 



Olfactory sac 



Olfactory bulb 

 Olfactory lobe 

 Cerebral 

 hemisphere 

 (telencephalon) 



Diencephalon 

 Optic lobe 

 (mesencephalon 

 Cerebellum 

 (mefencephalonj 



Medulla 

 oblongata 

 (myelencephalon) 

 Fourth ventricle 

 Semicircular 

 canals 



Glossopharyngea 

 IX 



Nerve (spinal) 

 cord 



Terminal 

 Olfactory tract i 



Pineal body 



-Superior 



oblique muscle 

 Optic II 



Trochlear IV 

 Anterior 



Rectus muscle 



-^ — Superior 



Oculomotor III 



Posterior 



rectus muscle 



Abducens VI 



Trigeminal V 



+ facial VII 



Spiracle 



-Auditory VIII 



Vagus X 

 Spinal nerve 1 



-■mm 



Gill cleft 



Oill 



Figure 248. Dorsal view of the brain and cranial nerves of the dogfish shark. Tl:e Roman 

 numerals are used to identify the cranial nerves. 



