>S'^iV^*S'^ ORGANS. 445 



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 pei-forations 

 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 tlie formation of the auditory involution 

 in Amphibia does to the same process in Birds. In both Elasmobranchii 

 and Amphibia the mucous layer of the epiblast behaves exactly as does 

 the whole c])iblast in the other types, but is shut off from the surface by 

 the passive epidermic layer of the epiblast. 



The miicous canals of the head and the ampullfe are formed from the 

 mucous layer of the ej)idermis in a manner very similar to the lateral line ; 

 but the nerves to them arise as simple branches of the fifth and seventh 

 nerves, which unite with them at a series of points, but do not follow 

 their course like the lateral nerve. 



It is clear that the canal of the lateral line is secondary, as compared 

 with the open groove of Chimsera or the segmentally arranged sense bulbs 

 of young Teleostei ; and it is also clear that the ph}'l()genetic mode of 

 formation of the canal consisted in (he closure of a primitively open groove. 

 The abbreviation of this process in Elasmobranchii was probably acquired 

 after the appearance of food-yolk in the egg, and the consequent dis- 

 appearance of a free larval stage. 



While the above points are fairly obvious it does not seem easy to 

 decide a j)Tiori whether a continuous sense groove or isolated sense bulbs 

 were the pi-imitive structures from which the canals ot the lateial line 

 took their origin. It is equally easy to picture the evolution of the canal of 

 the lateral line either from (1) a continuous unsegmented sense line, certain 

 points of which became segmentally differentiated into special sense bulbs, 

 while the whole subsequently formed a groove and then a canal ; or from 

 (2) a series of isolated sense bulbs, for each of which a protective groove 

 was developed ; and from the linear fusion of which a continuous canal 

 became formed. 



From the presence liowever of a linear streak of modified epidermis 

 in larval Teleostei, as well as in Elasmobranchii, it appears to me more 

 probable that a linear sense streak was the primitive structure from which 

 all the modifications of the lateral line took their origin, and that the 

 segmentally arranged sense bulbs of Teleostei are secondary differentiations 

 of this primitive structure. 



The, at first sight remarkable, distribution of the vagus nerve to the 

 lateral line is probably to be explained in connection with the evolution 

 of this organ. As is indicated both by its innervation from the vagus, 

 as also from the region Avhere it first becomes developed, the lateral line 

 was probably originally restricted to the anterior part of the body. As it 

 became prolonged backwards it naturally carried with it the vagus nerve, 

 and thus a sensory branch of this nerve has come to innervate a region 

 which is far beyond the limits of its original distribution. 



