COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 303 



Dogfish: Along the trunk the system consists of the lateral lines, which 

 mark the position of a canal. Find the lateral line on the head. Remove the 

 skin at this place, noting the underlying canal and the pores connecting the canal 

 with the surface. Trace the lateral line forward, removing the skin as you pro- 

 ceed. At the level of the spiracles the canals of the two lateral lines are connected 

 by the supratemporal canal. Anterior to this, each forks into a supraorbital 

 canal passing forward above the eye and an infraorbital canal passing ventrally 

 between the eye and the spiracle and then forward below the eye. Trace the 

 supraorbital canal to the end of the rostrum ; here it turns and proceeds pos- 

 . teriorly again parallel to its former course and becomes continuous with the 

 infraorbital canal. The latter gives off a hyomandibular branch running poste- 

 riorly along the sides of the jaws, and turns to the ventral surface of the ros- 

 trum, passing first posterior to the nostril and then turning forward between 

 the two nostrils. There is also a short mandibular canal under the skin just 

 behind the lower jaw; it is not connected with the other canals. 



Skate: The lateral line system is more complex than in the dogfish and 

 more difficult to follow. The lateral line canal runs on the dorsal surface just 

 lateral to the mid-dorsal spines. Remove the skin at this place and identify 

 the canal. Trace it forward, removing the skin as you proceed. At the pos- 

 terior end of the cartilage (propterygium) of the anterior part of the pectoral 

 fin it gives off two canals which proceed posteriorly over the surface of the fin. 

 It then proceeds above the eye as the supraorbital canal, apparently connecting 

 with its fellow by a cross-union on the posterior part of the skull. The supra- 

 orbital canal passes in front of the eye and as the infraorbital canal below the 

 eye. In the region of the eye it gives off branches over the rostrum and a long 

 branch which proceeds posteriorly along the lateral margin of the fin. On the 

 ventral side of the skate there is a prominent canal passing just lateral to the 

 gill slits. Trace this forward, noting branches behind and in front of the nostril 

 and on the ventral surface of the rostrum. On the surface of the pectoral fins 

 after removal of the skin the numerous, very long canals of Lorenzini are no- 

 ticeable. 



Draw, showing distribution of the canals. 



c) The olfactory organs: These consist of a pair of olfactory sacs on the ventral 

 side of the rostrum, opening externally by the nostril or external naris, with which 

 are associated various flaps of skin. Dissect the skin away from one olfactory 

 sac and cut away the flaps so that you can look into the sac. Note the numerous 

 plates or lamellae arranged in rows inside of the sac; these are covered with 

 olfactory epithelium, the sense of smell being quite keen in fishes. Prove to 

 yourself that the olfactory sac is closed internally, having no communication 

 with the oral cavity. 



Draw, showing the lamellae. 



