444 ORGANS OF THE LATERAL LINE. 



The organs of the lateral line consist as a rule of canals, partly situated 

 in the head, and partly in the trunk. These canals open at intervals ou 

 the surface, and their walls contain a series of nerve-endings. The 

 branches of the canal in the head are innervated for the most part by 

 the fifth pair, and those of the trunk by the nervus lateralis of the vagus 

 nerve. There is typically but a single canal in the trunk, the openings 

 and nerve-endings of which are segmentally arranged. 



Two types of development of these organs have been found. One of 

 these is characteristic of Teleostei ; the other of Elasmobranchii. 



In just hatched Teleostei, Schulze (No. 402) found that instead of the 

 normal canals there was present a series of sense bulbs, projecting freely 

 on the surface and partly composed of cells with stiff hairs. In most 

 cases each bulb is enclosed in a delicate tube op;^n at its free extremity ; 

 while the bulbs correspond in number with the myotomes. In some 

 Teleostei (Gobius, Esox, etc.) such sense organs persist through life ; in 

 most forms however each organ becomes covered by a pair of lobes of the 

 adjacent tissue, one formed above and the other below it. The two lobes 

 of each pair then unite and form a tube open at Vioth ends. The linear 

 series of tubes so formed is the commencement of the adult canal ; while 

 the primitive sense bulbs form the sensory organs of the tubes. The 

 adjacent tubes partially iinite into a continuous canal, but at their points 

 of apposition pores are left, which place the canal in communication with 

 the exterior. 



Besides these parts, I have found that there is present in the just hatched 

 Salmon a linear streak of modified epidermis on the level of the lateral 

 nerve, and from the analogy of the process described below for Elasmo- 

 branchii it appears to me probable that these streaks play some part in the 

 formation of the canal of the lateral line. 



In Elasmobranchii (Scyllium) the lateral line is formed as a linear 

 thickening of the mucous layer of the epidermis. This thickening is at 

 first very short, but gradually grows backwards, its hinder end forming a 

 kind of enlarged growing point. The lateral nerve is formed shortly after 

 the lateral line, and by the time that the lateral line has reached the level 

 of the anus the lateral nerve has grown back for about two-thirds of that 

 distance. The lateral nerve would seem to be formed as a branch of the 

 vagus, but is at first half enclosed in the modified cells of the lateral line 

 (fig. 275, niy, though it soon assumes a deeper position. 



A permanent stage, more or less corresponding to the stage just described 

 in Elasmobranchii, is retained in Chimsera^ and Echinorhinus spinosus, where 

 the lateral line has the form of an open groove (Solger, No. 404). 



The epidermic thickening, which forms the lateral line, is converted 

 into a canal, not as in Teleostei by the folding over of the sides, but by 

 the formation of a cavity between the mucous and epidermic layers of the 

 epiblast, and the subsequent enclosure of this cavity by the modified cells 

 of the mucous layer of the epiblast which constitute the lateral line. 

 The cavity first appears at the hind end of the organ, and thence extends 

 forwards. 



' Gotteand Semper both hold that the lateral nerve, insteai of growing in a centri- 

 fugal manner like other nerves, is directly derived from the epiblast of the lateral line. 

 For the reasons which prevent me accepting this view I must refer the reader to my 

 Monograph on EJasmohranch Fishes, pp. 141 — 146. 



