118 



VERTEBRATE ZOOLOGY 



Nervous System. Though the brain (Fig. 64) is very small, it is 

 larger in proportion to body size than that of the Cyclostomata. The 

 most striking feature is the large size of the olfactory bulbs. Cerebral 



hemispheres are well defined, cerebellum 

 is large and overlaps anteriorly a part of 

 the optic lobes and posteriorly a part of 

 the medulla oblongata. The region of 

 the thalamencephalon from which come 

 the optic nerves is comparatively small 

 and slender. The spinal cord is typical, 

 and inclosed within cartilaginous neural 

 arches. 



Sense Organs. The dominant sense 

 of the elasmobranch is olfactory; the 

 sense organ consisting of large con- 

 voluted invaginations in close contact 

 with the olfactory bulbs of the brain. 

 The eyes, though small and probably 

 not especially keen-sighted, are well de- 

 veloped and connected with the brain 

 by rather slender optic nerves. The 

 auditory organs are inclosed in cartila- 

 ginous capsules and consist of three 

 , ^circular canak, a utrMus, and a 



vew. 2, pineal stalk; 3, olfac- small simple sacculus. The lateral line 

 tory lobe; 4, cerebral hemi- sense organs are in grooves of the skin 



f^o tbttSS; <*> <**&<* d ^ *** divide *> 



10, roof of hind-brain; 11, 12, several branches in the head region, 



13, 14, muscles that move the O ne above and one below the eye and 



SSu&'S'Urii?; *> hyo-mandibular reg ion. 



17, main trunk of vagus nerve; The dog-fish represents neither ex- 



II-X, roots of the cranial nerves, treme of elasmobranch evolution, but 



primitive extinct sharks and the most 

 specialized modern skates and rays. It will be instructive to con- 

 sider some of the most primitive elasmobranchs in order to be able 

 to judge more certainly which of the characters of our favorite labo- 

 ratory fish are specialized and which are still primitive. A very prim- 

 itive form of living shark is the Frilled Shark, Chlameidoselachus (Fig- 



