digits, or is evidently a reduced form of the five-digit limb.*

(c) The absence of a median fin supported by fin rays.**

* The frog shows indications of a sixth digit.
** The frog's tadpole has a median fin, but no fin rays.

Section 147. The rabbit shares the following features with all the
vertebrata above the fishes and amphibia (= frogs, toads, newts,
and etc.)--

(a) Absence of gills (not gill slits, note) at any stage in
development.

(b) An amnion, and

(c) An allantois in development.

The meaning of (b) and (c) we shall explain to the student in the
chapters on embryology. We simply mention them here to render
our table complete.

Section 148. The rabbit shares with all mammals, and differs from all
other vertebrata (i.e., birds, reptiles, amphibia, and fishes), in having--

(a) Hair.

(b) A diaphragm.

(c) Only one aortic arch, and that on the left side of the body.

(d) Its young born alive. (But two very reptile-like mammals of
Australia, the duck-billed platypus and the echidna, lay eggs, and
certain fish and reptiles bear living young.)

(e) Epiphyses to its vertebral -centre- [centra].*

(f) The cerebral hemispheres covering the mid-brain.

(g) Corpora quadrigemina instead of bigemina.

[(h) A corpus callosum.]

[(i) A spirally coiled cochlea to the internal ear.]

[(In respect to h and i also, the echidna and platypus are scarcely
mammalinan.)]

* But certain mammals have no such epiphyses.

Section 149. The rabbit, together with the hares and conies, rats and
mice, voles, squirrels, beavers, cavies, guineapigs is included in that
order of the class of mammals which is called the rodentia, and is
distinguished by the character of the incisor teeth from other orders of
the class.

10. _Questions and Exercises_

1. Describe the venous circulation of the rabbit (with diagrams).
Compare a vein and artery. Compare the distribution of the great
venous trunks with that of the arterial system.

2. Construct a general diagram of the circulation of the rabbit, to show
especially the relation of the portal system, the lymphatics and
lacteals, and the renal circulation to the main blood current.

3. Draw the alimentary canal of the rabbit from memory.

4. What is a villus? Describe its epithelium, and the vessels within it.
Write as explicit an account as you can of the absorbent action of a
villus.

5.