30 THE PELAGIC STAGES OF YOUNG FISHES. 



supplied with branches from a single pair of nerves. In all respects, there- 

 fore, the segmental character of the sense-organs has been more perfectly 

 preserved in the head than in the trunk. These differences must, however, 

 be regarded as secondary in origin, for there can no longer be any doubt, 

 if any has existed since the investigations of Stannius* and of Solger.f that 

 these sense-organs are segmental. This point is conclusively established 

 by their developmental history, for a knowledge of which we are indebted 

 to the recent investigations of Beard. J In the case of Salmofario, according 

 to Beard, the lateral organs make their appearance first as a cord of cells 

 derived from the inner epiblastic stratum, extending from the region of the 

 neck along the whole length of the body at the height of the chorda. In 

 every segment are formed thickenings of this cord of cells, which give rise to 

 the sense-bulbs of the lateral line. For the first two or three months these 

 sense-bulbs are seen as protuberances on the free surface of the epidermis, 

 but soon after the disappearance of the yolk-sac they are shut off from the 

 surface by the development of the so-called " mucous canals." Many of the 

 primitive epiblastic thickenings are not carried to the stage of fully developed 

 sense-bulbs, but disappear in early stages by atrophy. It is in this way that 

 the metameric arrangement of the permanent bulbs becomes obscured. 



Starting then with the fact that the lateral-line organs are undoubted 

 segmental sense-organs, how are we to explain their position at different 

 heights ? Is this the result of a shifting of position, or may it be regarded 

 as an evidence that these organs were originally arranged in several rows on 

 each side of the body ? It has long been taken as a matter of course, that 

 the different patterns of arrangement were to be referred primarily to one, 

 in which there was only a single row on each side. The discovery of 

 such a type among the Annelids by Eisig has tended to confirm this idea. 

 This view will have now to be reconciled with the fact that in one large 

 group of Annelids, the Hirudinidie, there are from twelve to fourteen dis- 

 tinct rows of segmental sense-organs. There are some reasons for believing 

 that this arrangement is more primitive than that seen in the Capitellida3, but 

 they cannot be dealt with here. In case this suggestion be well founded, 

 the occurrence of more than one lateral line, as is the case in some fishes, 

 might be explained on other grounds than a multiplication by division. 



* Stannius. " Das peripherische Nervensystem dor Fisclie," Rostock, 1849. 



f Solger. "Neue tJntersuchungen zur Anatomic der Seitenorgane der Fisclie," Arch. I. Mik. Auat., 

 XVH. and XVI II., 1879-80. 



+ Beard. Zoolog. An/.., VII., No. 161, 1884, p. 12:! 



