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, Ze- 
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, J, 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 S 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, 5S, 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 Chimera. 
+ On the Lateral Line System of Amia calva. F. of Morph., 1889. 
s : 
