28 THE PELAGIC STAGES OF YOUXG FISHES. 



The Sense-organs of the Lateral Line. — There are one or two points relat- 

 ing to the structure and distribution of these organs to which we would here 

 briefly call attention, since, they appear to bear directly on the questions of 

 their morphological and physiological significance. These organs are usually 

 described as furnished with delicate hairs, which certainly gives a very inad- 

 equate notion of the picture they present in the living fish. Instead of a 

 tuft of distinct hairs, we find a very delicate and perfectly transparent mem- 

 branous extension, of such extreme tenuity that it presents only a shadowy 

 outline. It is only in the base of this membranous extension that hair-like 

 thickenings are recognizable; and these thickenings, tapering outward, van- 

 ish at a short distance from the rounded summit of the sense-bulb, beyond 

 which point there is not the faintest indication of hair-like structui'e, and 

 only now and then a minute granule to break the almost perfect homoge- 

 neity of the membrane. While this terminal portion of the sense-organ 

 may be described as a strap-shaped appendage ("cupula terminalis," Solger), 

 somewhat resembling the tiny locomotive flappers of a Ctenophore. it is to be 

 remembered that it becomes thicker and more columnar at its base. If one 

 of the sense-organs be viewed en face, it will be seen that the hair-like thick- 

 enings of the columnar portion of the appendage are evenly distributed over 

 its summit. Ontogenetieally considered, this appendage may be regarded 

 as a tuft of coalesced sense-hairs. 



Our description of it is based upon a careful examination of the struc- 

 ture in living and healthy specimens. In specimens injured by pressure 

 or otherwise, the more delicate, thinner portion of the appendage is soon 

 lost, leaving the basal portion with its thickenings looking more or less like 

 disconnected hairs. 



In this species there are four pairs of these sense-organs symmetrically 

 placed on the sides of the head, and from four to five pairs on the bodv, of 

 which usually only the first two are metamerically disposed, the rest being, 

 strictly speaking, unpaired, those of one side alternating at longer or shorter 

 intervals with those of the other side. The first pair is situated in front of 

 and between the olfactory pits; the second, a little in advance of the eyes. 

 and somewhat above the middle of the head in a vertical direction; the 

 third, still higher up on the sides of the head, at some distance above the 

 posterior portion of the eye; the fourth, a little lower down, barely above 

 the posterior edge of the eye; the fifth, at the beginning of the second third 

 of the total length, high above the plane of the lateral nerve; the sixth, 



