274 



THE ELASMOBRANCH FISHES 



DEVELOPMENT OF EAR 



The ear {Scyllium, fig. 239), like the nose, forms as a pit. In the development 

 of the ear, however, the vesicle thus formed sinks in and, as the sacculus (s.) , 

 becomes far removed from the exterior. It does not, however, lose entire con- 

 nection with the outside for as it sinks inward it becomes flask-shaped, the 



long neck being the endolymphatic 

 duct {e.d.). At this stage the outer 

 wall of the vesicle becomes thin and 

 the anterior oblique and horizontal 

 semicircular canals {aos. and hr.) de- 

 velop from them. 



Sensory Canal System and 



Ampullary and Pit 



Organs 



The sensory canal system as we have 

 seen in Heptanchus consists of exten- 

 sive sensory canals over the head and 

 along the side of the body. The am- 

 pullary organs associated with cer- 

 tain of the canals and innervated by 

 the same nerves are confined to the 

 region of the head. Certain modifica- 

 tions of the latter, the vesicles of Savi, 

 may also be present. The pit organs 

 are mainly in the anterodorsal trunk 

 region but some of them are in the 

 segment of the head. 



Sensory Canal System 



The sensory canals take a general 

 course parallel to the long axis of the 

 body. In the region of the trunk and 

 tail thej^ compose the lateral line ob- 

 served in our study of external form, 

 and in the region of the head they 

 form the cephalic canals, three or four 

 main divisions of which are present in 

 the sharks. One of these, the supra- 

 orbital canal {soc, figs. 240-242) , runs above the eye; another, the infraorbital 

 (ioc), passes back of and forward below the eye; the third or hyomandibular 



Fig. 240. Dorsal view of cephalic canals in 

 Laemargus. (From Ewart.) (Drawn as 

 transparent object.) 



cc, commissural or supratemporal canal ; 

 lime, hyomandibular canal; ioc, infraorbi- 

 tal canal; ZL, lateral canal; soc, supraorbital 

 canal. 



