LATERAL LINE ORGANS 



51 



sensory canals suggests the modifications to which the 

 open sensory groove has been subjected. There are thus 

 forms in which the canal becomes more and more deeply 

 sunken in the integument, and acquires a tubular char- 

 acter by the fusing together of its outer margins. The 

 section of the lateral line of the Greenland shark, L(E- 

 margus (Fig. 62, v. p. 90), shows the tube-like sensory 

 canal well sunken from the surface, but retaining met- 

 ameral openings at the points. The sensory cells, S, 

 are no longer, as in Fig. 61, scattered evenly along the 

 floor of the canal ; they now occur in metameral masses 

 supplied with a distinct nerve branch, N, located in the 

 region immediately below the external tubules. When 

 sunken in the integument, the sensory canal is known to 

 have acquired supporting structures to enable its tubular 

 character to be maintained ; in the Cretaceous shark, 

 Mesiteia, an elaborate series of surrounding calcified rings * 

 were thus evolved. 



Further changes in the mucous canal are often accom- 

 panied by the subdivision of the external apertures ; each 

 of the openings of Fig. 62 might by this process give rise 

 to a series of minute surface pores, as at 6" in Fig. 65, or 

 enlarged, showing the collecting mucous canals in Fig. 66. 

 This ramose mode of termination of the external tubules 

 has been admirably described by Allis f in the ontogeny of 

 a ganoid ; in a larval stage (Fig. 64, S, S, S), the condi- 

 tion of the sensory canals is seen to differ little from 

 those shown in section in Fig. 62 ; although imbedded 

 in the integument, occasional pores are seen, 5, S, to 

 open to the surface ; these subsequently by repeated sub- 

 division give rise to the great number of minute open- 



* A condition somewhat similar has been noted (Leydig) in Chimsera. 

 t On the Lateral Line System of Amia calva. y. of Morph., 1889. 



