124 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER VII. 



SOMATIC AFFERENT DIVISION. SPECIAL 

 CUTANEOUS SUBDIVISION. 



The typical sense organs of this system are the pit and canal 

 organs which are found in rows on the head and along the lateral 

 line of cyclostomes, fishes and aquatic amphibia. In Figure 6i 

 are shown a large and a small organ of this type from a sucker 

 embryo at about the time of hatching. The organ consists of 

 high columnar supporting cells which form the whole thickness 

 of the epidermis within the area of the organ, and of shorter 

 thicker pear-shaped cells which do not reach the whole depth 

 of the epidermis. The latter cells bear at their outer ends cuticular 



Fig. 6i. — A large and a small neuromast from a sucker {Catostomus) embrj'O at 

 about the time of hatching. 



hairs or bristles which project beyond the surface. These are 

 the sense cells. Beneath the organ a few nerve fibers come up 

 from a deeper lying nerve, lose their medullary sheaths as they 

 reach the organ and penetrate between the cells. Here the fibers 

 divide in a very comphcated manner (Bunker) and end by very 

 fine branches on the surface of the cells. In the typical pit organs 

 the surrounding epidermis is much thicker and rises up as a wall 

 on all sides, the bottom of the pit being formed by the sense organ. 

 Organs of this type are found in fishes in a line along the lateral 

 surface of the body, and in a supraorbital, an infraorbital and a 

 hyomandibular row (Fig. 62). The organs of these typical rows 

 in fishes are usually enclosed in canals in the manner described 

 in the chapter on embryology, but in cyclostomes and amphibia 



