ic duct (p.d.), and which we have figured under that
name, is always developed. Anteriorly it opens into the body cavity. It
is also called the Mullerian duct, and in the great majority of
vertebrata it becomes the oviduct, uniting with its fellow, in the case of
the dog-fish, ventral to the oesophagus. In the male it usually
disappears; the uterus masculinus of the rabbit is still very generally
regarded as a vestige of it. Kolliker has shown, however, that this
interpretation is improbable. Ms. is the mesonephros, some or all of
which becomes the epididymis in the male of types possessing that
organ, and is connected with G. by the vasa efferentia. Mt., the
metanephros, is, in -actual fact- [the frog], indistinguishably
continuous with Ms., and is the functional kidney, its duct
(metanephric duct) being either undifferentiated from the mesonephric
(as is the case with the frog) or largely split off from it, as in the
dog-fish, to form the ureter.

Section 20. The correspondence of the male organs of the dog-fish
with those of the rabbit, will be more evident if the student imagine--

(a) the testes, vasa efferentia, and epididymis of each side to
shift posteriorly until they reach a position on either side of
the cloaca; and

(b) The uro-genital apertures, instead of meeting dorsally and
posteriorly to the anus, to shift round that opening and meet
anteriorly and ventrically to it.

Section 21. This completes our survey of this type. Except where we
have specified differences, the general plan of its anatomy follows the
lines of the other vertebrate types described.

_Questions on the Dog-Fish_

1. Describe the alimentary canal of the dog-fish, and compare it with
that of the rabbit in detail.

2. Compare the coelom of the dog-fish and rabbit.

3. Draw diagrams to illustrate the course of the circulation in the
dog-fish.

4. (a) Describe fully the heart of a dog-fish. (b) Compare it with that of
a rabbit.

5. Give an account of the respiratory apparatus of the dog-fish.

6. Draw diagrams of a dog-fish vertebra, and compare the centrum
with that of a rabbit.

7. Compare the vertebral column of the dog-fish and rabbit.

8. Draw diagrams of the limbs and limb-girdles of the dog-fish.
Compare the pectoral with the pelvic fin.

9. Draw diagrams of (a) the male and (b) the female urogenital organs
of the dog-fish. (c) Compare them carefully with those of the rabbit.

10. Compare