SPECIAL CUTANEOUS SUBDIVISION. 



125 



they remain in the form of pit organs. In many cases accessory 

 lines of pit organs, or even canals, are present and in selachians 

 the total number of organs is very large. The number of organs 

 found in the typical rows of the head varies greatly, as is indicated 

 by the following examples: Amia, 40; Menidia, 37; Gadus, 28; 

 Chimaera, 93. It has recently been shown by experiment that 

 the function of the pit and canal organs is to take account of 

 vibrations in the water of a frequency too low for the production 

 of sound (between 6 and 100 per second). 



. ophthal. superfii 



Supraorbital 

 canal 



Hyomand 

 canal 



Infraorbital canal 



Lateral line 

 canal 



h>'omandibularis 



Fig. 62. — A diagram of the lateral line canals and nerves in the ganoid fish, 

 Amia calva. After E. Phelps Allis. 



In selachians there are found two other forms of sense organs 

 which are histologically similar to the pit and canal organs and 

 are also related to them in their nerve supply. These are the organs 

 known as the ampullae of Lorenzini and the vesicles of Savi. The 

 ampullae are found in groups including in some cases a large 

 number of organs, imbedded in a soft gelatinous connective 

 tissue in cavities about the skull or between other organs. The 

 bodies which are imbedded in these groups, however, are only 

 the enlarged ends of slender canals which after a longer or shorter 

 course open on the surface. The development of the organs shows 



