SENSE ORGANS. 539 



cases each bulb is enclosed in a delicate tube open 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 both 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 unite 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, nl) 1 , though it soon assumes a deeper position. 



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

 in Elasmobranchii, is retained in Chimasra, 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. 



After its conversion into a canal the lateral line gradually recedes from 

 the surface ; remaining however connected with the epidermis at a series 

 of points corresponding with the segments, and at these points perforations 

 are eventually formed to constitute the segmental apertures of the system. 



The manner in which the lumen of the canal is formed in Elasmo- 

 branchs bears the same relation to the ordinary process of conversion 

 of a groove into a canal that the formation of the auditory involution 



1 Gotte and Semper both hold that the lateral nerve, instead of growing in a 

 centrifugal 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 Elasmobranch Fishes, pp. 141 146. 



