4 



spiracles, are two pores, the external openings of the endolymphatic 

 ducts which communicate with the internal ear. Large numbers of 

 smaller pores can be found on all surfaces of the head, some in 

 groups, some arranged linearly, many scattered. Most of those 

 arranged in lines lead into the sensory canal system which continues 

 from the lateral line canal upon the head, while the majority of 

 the others belong to a separate type of sense organs, the ampullae 

 of Lorenzini. 



Make a cut encircling the pores of the endolymphatic ducts and 

 close to them. Do not remove this piece of skin. From it make a 

 median incision forward to the tip of the snout and back as far as 

 the level of the first gill slit. Starting at this incision work the 

 skin off from the tissues beneath it. This must be a careful, close 

 dissection. When the lateral line is reached it will be seen that the 

 lateral line canal is continued upon the head and is joined by sev- 

 eral others. By looking through the loosened skin toward the light 

 the pores can be seen which lead from the canals to the surface. In 

 the hollow of the skull in front of and above the eye is a large 

 group of tubules which open through the pores so conspicuous at 

 this point. At the internal end of each tubule is a slight enlarge- 

 ment, of denser tissue, with which a delicate nerve strand can often 

 be seen connected. The nerve strands can be traced to a large 

 nerve passing above the eye and distributed to the snout. These 

 tubules are the ampullae of Lorenzini. This group of ampullae, 

 which may be called the dorsal group, is quite definitely demarked. 

 It will be noted that the inner ends of the ampullae are grouped in 

 a much smaller area than their pores. 



Between the spiracle and the first gill slit will be found a lateral 

 group of similar organs. Notice the arrangement of their tubules 

 and pores. Under the snout are two groups of ampullae on each 

 side of the midline. The inner ventral group is separated from the 

 outer ventral group by the lateral bar of the rostral cartilage. Some 

 of the tubules of the outer ventral group will be found to extend to 

 pores situated at the sides of and behind the mouth. 



An adult fish possesses from 1200 to 1900 ampullae of Loren- 

 zini. Their function is not well understood, but it has been sug- 

 gested that they are organs responsive to stimuli of pressure, either 

 of currents or water, or resulting from depth, or even of deep 

 tones. 



The system of sensory canals consists of the following members 

 on each side of the head: 



A supraorbitaly passing above the eye to the end of the snout 

 and bending back on the ventral surface to join the infra-orbital. 



An infra-orbital., which branches off from the supra-orbital and 

 passes ventrad between the eye and the spiracle, then turns forward 

 along the ventral margin of the orbit, and finally bends toward the 

 mid-line and extends to the tip of the snout. 



A hyomandibular, which leaves the infra-orbital below the eye 

 and runs back beyond the angle of the mouth. 



A short mandibular, on the mandible close to the angle of the 

 mouth, which is not connected with the other canals. 



The canal systems of the right and left sides are connected by a 

 supra-temporal canal just behind the endolymphatic pores, and 

 often by an anastomosis of the infra-orbital canals in front of the 

 mouth. 



On some specimens two crescentic rows of pores can be found 

 between the bases of the pectoral fins, which represent a third type 



