otic vesicle 



and endolymphatic duct 

 spiracular placode 

 supraorbital line 



intertemporal 

 glossophoryngeal placode 

 vagus placode 



Ist branchial opening 



hyoid arch line 



mandibular arch line 

 spiracular opening 



B 



supratemporol orgon 

 tabulor organ 



endolymphatic pore 



extroscopular 

 placodes 



hyoid arch line 

 ondibulor line (jugol and 

 oral divisions 



temporal division 



spiracular organ 



hyoid arch line 

 mandibular arch line 



nasal openmg 



Figure 13-28. Three stages in the development of the sensory-line system of Squo/us acanthios. 

 (After Holmgren and Pehrson, 1949) 



end of the supraorbital line of the shark. The infraorbital is 

 then peculiar because of its relationship to the olfactory 

 capsule and also its forking (only in Hydrolagus); both parts 

 connect across the midline, and the upper also connects 

 along the midline with the supraorbital connective. 



The modifications of these lines may be related to the 

 jaw changes and the development of the snout. The chimae- 

 rid differs from the shark in terms of the position of its nasal 

 capsule much as the dipnoan differs from the other choa- 

 nates. 



Fossil gnaihostome fishes In the arthrodires and acantho- 

 dians, the same general canal pattern is seen. In the acan- 

 thodian the canals lie in the scales covering the head. In 

 one genus, Poracanthodes, there is a reticular canal system 

 embedded in the scales. There are distinct mandibular and 

 hyoid arch lines which join at the angle of the mouth, and 

 there is a pit line of the cheek which joins the jugal section 

 with the preopercular section. The preopercular section lies 

 far forward of the spiracle and does not connect with the 

 temporal line. The position of this line indicates that these 



nasal opening 



Figure 13-29. Sensory-line grooves on the head of Hydrolagus. A, lateral view; B, interconnec- 

 tions on snout. 



414 • THE NERVOUS SYSTEM 



