The Evolution of Man, vol 2 | Page 5

animals; it was a very fortunate accident that the embryology
of these related forms was discovered just at the time when the question
of the descent of the vertebrates from the invertebrates came to the
front. In order to understand it properly, we must first consider these
remarkable animals in their fully-developed forms and compare their
anatomy.
We begin with the lancelet--after man the most important and
interesting of all animals. Man is at the highest summit, the lancelet at
the lowest root, of the vertebrate stem.
It lives on the flat, sandy parts of the Mediterranean coast, partly buried
in the sand, and is apparently found in a number of seas.* (* See the
ample monograph by Arthur Willey, Amphioxus and the Ancestry of
the Vertebrates; Boston, 1894.) It has been found in the North Sea (on
the British and Scandinavian coasts and in Heligoland), and at various
places on the Mediterranean (for instance, at Nice, Naples, and
Messina). It is also found on the coast of Brazil and in the most distant
parts of the Pacific Ocean (the coast of Peru, Borneo, China, Australia,
etc.). Recently eight to ten species of the amphioxus have been

determined, distributed in two or three genera.
(FIGURE 2.210. The lancelet (Amphioxus lanceolatus), twice natural
size, left view. The long axis is vertical; the mouth-end is above, the
tail-end below; a mouth, surrounded by threads of beard; b anus, c
gill-opening (porus branchialis), d gill-crate, e stomach, f liver, g small
intestine, h branchial cavity, i chorda (axial rod), underneath it the aorta;
k aortic arches, l trunk of the branchial artery, m swellings on its
branches, n vena cava, o visceral vein.
FIGURE 2.211. Transverse section of the head of the Amphioxus.
(From Boveri.) Above the branchial gut (kd) is the chorda, above this
the neural tube (in which we can distinguish the inner grey and the
outer white matter); above again is the dorsal fin (fh). To the right and
left above (in the episoma) are the thick muscular plates (m); below (in
the hyposoma) the gonads (g). ao aorta (here double), c corium, ec
endostyl, f fascie, gl glomerulus of the kidneys, k branchial vessel, ld
partition between the coeloma (sc) and atrium (p), mt transverse 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 neither vertebrae
nor skull nor limbs (Figures 1.83 to 1.86). And even after the formation
of the primitive vertebrae has begun, the segmented foetus of the
amniotes still has for a long time the simple form of a lyre-shaped disk
or a sandal, without limbs or extremities. When we compare this
embryonic condition, the sandal-shaped foetus, with the developed
lancelet, we may say that the amphioxus is, in a certain sense, a
permanent sandal-embryo, or a permanent embryonic form of the
Acrania; it never rises above a low grade of development which we
have long since passed.
The fully-developed lancelet (Figure 2.210) is about two inches long, is
colourless or of a light red tint, and has the shape of a narrow
lancet-formed leaf. The body is