No. 3-] LATERAL LINE OF AMIA. 537 
16 and 17. These two anastomoses, and those of the anterior 
commissure with the infra-orbital, and the supratemporal com- 
missure with the corresponding canal of the other side, are the 
result of the fusion of two primary pores which coalesce to 
form double pores, in the same way that two half-pores unite 
to form the primary ones. Double pores are formed in this 
way, and then disappear at the point where the anterior com- 
missure joins the canal of the other side, and where the supra- 
temporal commissure joins the main infra-orbital. One other 
anastomosis, formed in a somewhat different way between 
systems 6 infra-orbital and 4 supra-orbital, establishes a second 
connection between these two canals, and completes the circuit 
of the orbit. 
There are on each side three lines of pit organs on top of the 
head, besides two on the cheek, one on the mandible, and one 
on the gular plate. The anterior and middle pit lines on top of 
the head lie nearly above the anterior and posterior semicircular 
canals. On the body there are two series of pit lines, one on 
the back and one accompanying the lateral canal. 
In the adult the main canals lie in the deeper layers of the 
dermal bones. In larval stages they lie in open channels, or in 
bony tubes on the upper surface of these bones. Although 
some of the primary tubes issue through the bone, one always 
issues between every two consecutive bones along each line. 
The sense-organs always lie inside the bones. They are at 
first single, but by the formation of bud-like organs at each 
end of the original one, large groups are formed, which in 
certain stages resemble the nasal epithelium of Exocetus and 
other forms, as given by Blaue. 
The nasal pits are inclosed in the same way that the lateral 
canals are, and the short canal so formed is at first continuous 
with the canal inclosing organ 5 infra-orbital. 
The head, gill-covers, and gular plate are thickly covered with 
the surface sense-organs called by Merkel terminal buds, which 
extend also onto the body. They are innervated in large part 
by the trigeminal, but probably also by the ophthalmicus pro- 
fundus, facialis, glossopharyngeus, and vagus. 
