terize the higher as contrasted with the
lower vertebrata.

15. Describe the skeleton of the upper and lower jaw (a) in the frog,
(b) in the rabbit. Point out exactly what parts correspond with one
another in the two animals compared. (c) What bone in the rabbit is
generally regarded as corresponding to the quadrate cartilage of the
frog?

-The Dog-Fish_

1. _General Anatomy_

Section 1. In the dog-fish we have a far more antique type of
structure than in any of the forms we have hitherto considered.
Forms closely related to it occur among the earliest remains of
vertebrata that are to be found in the geological record. Since the
immeasurably remote Silurian period, sharks and dog-fish have
probably remained without any essential changes of condition, and
consequently without any essential changes of structure, down to
the present day. Then, as now, they dominated the seas. They
probably branched off from the other vertebrata before bone had
become abundant in the inner skeleton, which is consequently in their
case cartilaginous, with occasional "calcification" and no distinct
bones at all. Unlike the majority of fish, they possess no swimming
bladder-- the precursor of the lungs; but in many other respects,
notably in the uro-genital organs, they have, in common with the
higher vertebrata, preserved features which may have been disguised
or lost in the perfecting of such modern and specialized fish as, for
instance, the cod, salmon, or herring.

Section 2. Comparing the general build, of a dog-fish with that of a
rabbit, we notice the absence of a distinct neck, and the general
conical form; the presence of a large tail, as considerable, at first, in
diameter as the hind portion of the body, and of the first importance in
progression, in which function the four paddle-shaped limbs, the
lateral fins, simply co-operate with the median fin along the back for
the purpose of steering; and, as a consequence of the size of the
tail, we note also the ventral position of the apertures of the body. The
anus, and urinary and genital ducts unite in one common chamber,
the cloaca. Behind the head, and in front of the fore fin (pectoral fin),
are five gill slits (g.s.) leading from the pharynx to the exterior. Just
behind the eye is a smaller and more dorsal opening of the same
kind, the spiracle (sp.). On the under side of the head, in front of the
mouth, is the nasal aperture (olf.), the opening of the nasal sac,
whi