hen the student commences to dissect the head of
a dog-fish he notices over the dorsal surface of the snout an exudation
of a yellowish jelly-like substance, and on removing the tough skin
over this region and over the centre of the skull he finds, lying beneath
it, a quantity of coiling simple tubuli full of such yellowish matter.
These tubuli open on the surface by small pores, and the nerves
terminate in hair-like extremities in their lining. These sense tubes
are peculiar to aquatic forms; allied structures are found over the head
and along a lateral line (see below) in the tadpole, but when the frog
emerges from the water they are lost. They, doubtless, indicate some
unknown sense entirely beyond our experience, and either only
possible or only necessary when the animal is submerged.

In addition to the ophthalmic moiety mentioned above, the seventh
nerve has a vidian branch (vid.) running over the roof of the mouth, and
besides this its main branches fork over the spiracle, just as V. forks
over the mouth, and as IX. and X. fork over gill clefts. This nerve in the
rabbit is evidently considerably modified from this more primitive
condition.

The eighth is the auditory nerve, as in the rabbit.

The ninth nerve forks over the first branchial cleft.

The tenth nerve is easily exposed by cutting down through the body
wall muscles over the gill clefts, into the anterior cardinal sinus
(A.C.S.). It gives off (a) branches forking over the posterior four gill
slits, (b) a great lateral nerve running inward, and back through the
body-wall muscle, and connected with a line of sense organs similar
to those in the head, the lateral line, and (c) a visceral nerve curving
round to the oesophagus and stomach. In dissection it becomes very
evident that the tenth nerve is really a leash of nerves, each one
equivalent to the ninth.

We may here call the attention of the reader to the fact of the singular
resemblance of V., VII., IX., and the factors of X. That each has a
ventral fork, we have already noticed. Each also (?IX.) has a dorsal
constituent connected with the sense organs of the skin. The vidian
branch of VII., however, is not evidently represented in the others.

Section 16. The coelom of the dog-fish is peculiar-- among the types
we treat of-- in the possession of two direct communications with the
exterior, in addition to the customary indirect way through the oviduct.
These are the abdominal pores (a.p.) on ei