22 The Exterior of the Fish 
condition of the lateral line is seen in the sharks and chimeras, 
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 porosissimus (Cuv. 
and Val.). Gulf of Mexico. 
with mucus. Analogous to these cavities are the mucous chan- 
nels which in primitive fishes constitute the lateral line. 
Function of the 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 
* Fishes Recent and Fossil, p. 52. 
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