NERVOUS SYSTEM, SENSES, AND SENSE ORGANS 199 



sign, as it were, of an underlying system of sense organs (Fig. 44). 

 These were probably evolved, in the first place, from simple 

 pit organs similar to those already described as being scattered 

 all over the skin, since their mode of development is the same. 

 The simplest type of lateral line system is found in such a 

 primitive Selachian as the Frilled Shark [Chlamydoselachus) ^ in 

 which the sense organs lie in a row in an open groove running 

 from the head to the tail, which is partially roofed over by the 

 bordering denticles in the skin (Fig. 79a). In most of the other 



Fig. 79. LATERAL LINE. 



a. Portion of lateral line of Frilled Shark {Chlamydoselachus anguineus), much 

 enlarged ; b. Scales of the Bow-fin {Aniia calva), showing apertures of lateral 

 line tubules ; c. Lateral line scale of Bow-fin {Amia calva), greatly enlarged ; 

 d. Vertical longitudinal section through lateral line of Perch {Perca fluviatilis), 

 much enlarged and diagrammatic; e. Lateral line scales of Osteoglossid (Heterotis 

 niloticus), x\. (a, b, and c after Bashford Dean.) 



Selachians this groove becomes converted into a tunnel sunk 

 beneath the skin, which communicates at regular intervals with 

 the surface by a series of pores, alternating with the sense organs 

 developed on the inner walls of the tube. In the embryo fish 

 these organs first appear on the surface of the skin, then sink 

 into a groove, and finally become enclosed in a tube beneath 

 the skin. Professor Cunningham has compared the lateral line 

 of a Dog-fish to a tube railway, the external apertures corre- 

 sponding to the stations at which the tubular tunnel communi- 

 cates with the surface of the ground by means of vertical shafts. 



