ventral muscle, n renal canals, of upper and uf lower canals in the
mantle-folds, p peribranchial cavity, (atrium), sc coeloma (subchordal
body-cavity), si principal (or subintestinal) vein, sk perichorda
(skeletal layer).)

Johannes Muller classed the lancelet with the fishes, although he
pointed out that the differences between this simple vertebrate and
the lowest fishes are much greater than between the fishes and the
amphibia. But this was far from expressing the real significance of
the animal. We may confidently lay down the following principle: The
Amphioxus differs more from the fishes than the fishes do from man and
the other vertebrates. As a matter of fact, it is so different from
all the other vertebrates in its whole organisation that the laws of
logical classification compel us to distinguish two divisions of this
stem: 1, the Acrania (Amphioxus and its extinct relatives); and 2, the
Craniota (man and the other vertebrates). The first and lower division
comprises the vertebrates that have no vertebrae or skull (cranium).
Of these the only living representatives are the Amphioxus and
Paramphioxus, though there must have been a number of different
species at an early period of the earth's history.

Opposed to the Acrania is the second division of the vertebrates,
which comprises all the other members of the stem, from the fishes up
to man. All these vertebrates have a head quite distinct from the
trunk, with a skull (cranium) and brain; all have a centralised heart,
fully-formed kidneys, etc. Hence they are called the Craniota. These
Craniotes are, however, without a skull in their earlier period. As we
already know from embryology, even man, like every other mammal,
passes in the earlier course of his development through the important
stage which we call the chordula; at this lower stage the animal has