The Exterior of the Fish 



23 



condition of the lateral line is seen in the sharks and chimaeras, 

 in which fishes it appears as a series of channels in or under 

 the skin. These channels are filled with mucus, which exudes 

 through occasional open pores. In many fishes the bones 

 of the skull are cavernous, that is, provided with cavities filled 



Fig. 15. — Singing Fish (with many lateral lines), Porichthys porosissitmis (Cuv. 

 andVal.). Gulf of Mexico. 



with mucus. x\nalogous to these cavities are the mucous chan- 

 nels which in primitive fishes constitute the lateral line. 



Function of tJie Lateral Line. — The general function of the 

 lateral line with its tubes and pores is still little understood. 

 As the structures of the lateral line are well provided with 

 nerves, it has been thought to be an organ of sense of some 

 sort not yet understood. Its close relation to the ear is beyond 

 question, the ear-sac being an outgrowth from it. 



"The original significance of the lateral line," according to 

 Dr. Dean,* "as yet remains undetermined. It appears inti- 

 mately if not genetically related to the sense-organs of the head 

 and gill region of the ancestral fish. In response to special 

 aquatic needs, it may thence have extended farther and farther 

 backward along the median line of the trunk, and in its later 

 differentiation acquired its metameral characters." In view 

 of its peculiar nerve-supply, "the precise function of this entire 

 system of organs becomes especially difficult to determine. 

 Feeling, in its broadest sense, has safely been admitted as its 

 possible use. Its close genetic relationship to the hearing 

 organ suggests the kindred function of determining waves of 

 vibration. These are transmitted in so favorable a way in 

 the aquatic medium that from the side of theory a system of 



