nd mouths, and which are divided into hags and
lampreys. The hags (myxinoides) have long cylindrical worm-like bodies.
The lampreys (petromyxontes) includes those well known "nine eyes"
common at the seaside.

These single-nostril animals (monorrhina) arose during the primordial
period out of the skull-less animals by the anterior end of the dorsal
marrow developing into the brain, and the anterior end of the dorsal
skull into the skull. By the division of the single nostril of the
members of the last group into two lateral halves, by the formation of a
sympathetic nervous system, a jaw skeleton, a swimming bladder and two
pairs of legs (breast fins or fore-legs, and ventral fins or
hind-legs), arose the primaeval fish (selachii), which is best
represented by the still-living shark (squalacei).

Out of the primaeval fish arose the mud-fish (dipneusta), which is very
imperfectly represented by the still-living salamander fish; the
primaeval fish adapting itself to land, and by the transforming of the
swimming bladder into an air-breathing lung, and of the nasal cavity
(which was now open into the mouth cavity) into air-passages. Their
organization _might_, in some respect, be like the ceratodus and
proloptems; but this is not certain.

The dipneusta is an intermediate stage between the selachii and
amphibia. Out of the dipneusta arose the class of amphibia, having five
toes (the pentadactyla). The gill amphibians are man's most ancient
ancestors of the class amphibia. Besides possessing lungs as well as the
mud-fish, they retain throughout life regular gills like the
still-living proteus and axolotl. Most gilled batrachia live in North
America. The paddle-fins of the dipneusta changed into five-toed legs,
which were afterwards transmitted to the higher vertebrata up to man.

The gilled amphibia (sozobrachia) of the last group finally lost their
gills but retained their tail, and tailed amphibians (sozura) were
produced, such as the salamander and newt of the present day. Out of the
sozura originated the primaeval amniota (protamnia) by the complete loss
of the gills by the formation of the amnion of the cochlea, and of the
round window in the auditory organ, and of the organ of tears. Out of
the protamnia originated the primary mammals (promammalia). The most
closely related were the ornithostoma; they differed through having
teeth in their jaws.

No fossil remains of the primary mammals have as yet been found,
alt