The Evolution of Man, vol 2 | Page 9

branchial gut, l liver-tube (on the right, one-sided), m
muscles, n renal canals, r spinal cord, sn spinal nerves, sp gill-clefts.)
In order to see this quite clearly, it is particularly useful to compare the
Amphioxus with the youthful forms of those vertebrates that are
classified next to it. This is the class of the Cyclostoma. There are
to-day only a few species of this once extensive class, and these may be
distributed in two groups. One group comprises the hag-fishes or
Myxinoides. The other group are the Petromyzontes, or lampreys,
which are a familiar delicacy in their marine form. These Cyclostoma
are usually classified with the fishes. But they are far below the true
fishes, and form a very interesting connecting-group between them and
the lancelet. One can see how closely they approach the latter by
comparing a young lamprey with the Amphioxus. The chorda is of the
same simple character in both; also the medullary tube, that lies above
the chorda, and the alimentary canal below it. However, in the lamprey
the spinal cord swells in front into a simple pear-shaped cerebral
vesicle, and at each side of it there are a very simple eye and a
rudimentary auditory vesicle. The nose is a single pit, as in the
Amphioxus. The two sections of the gut are also just the same and very
rudimentary in the lamprey. On the other hand, we see a great advance
in the structure of the heart, which is found underneath the gills in the

shape of a centralised muscular tube, and is divided into an auricle and
a ventricle. Later on the lamprey advances still further, and gets a skull,
five cerebral vesicles, a series of independent gill-pouches, etc. This
makes all the more interesting the striking resemblance of its immature
larva to the developed and sexually mature Amphioxus.
While the Amphioxus is thus connected through the Cyclostoma with
the fishes, and so with the series of the higher vertebrates, it is, on the
other hand, very closely related to a lowly invertebrate marine animal,
from which it seems to be entirely remote at first glance. This
remarkable animal is the sea-squirt or Ascidia, which was formerly
thought to be closely related to the mussel, and so classed in the
molluscs. But since the remarkable embryology of these animals was
discovered in 1866, there can be no question that they have nothing to
do with the molluscs. To the great astonishment of zoologists, they
were found, in their whole individual development, to be closely
related to the vertebrates. When fully developed the Ascidiae are
shapeless lumps that would not, at first sight, be taken for animals at all.
The oval body, frequently studded with knobs or uneven and lumpy, in
which we can discover no special external organs, is attached at one
end to marine plants, rocks, or the floor of the sea. Many species look
like potatoes, others like melon-cacti, others like prunes. Many of the
Ascidiae form transparent crusts or deposits on stones and marine
plants. Some of the larger species are eaten like oysters. Fishermen,
who know them very well, think they are not animals, but plants. They
are sold in the fish markets of many of the Italian coast-towns with
other lower marine animals under the name of "sea-fruit" (frutti di
mare). There is nothing about them to show that they are animals.
When they are taken out of the water with the net the most one can
perceive is a slight contraction of the body that causes water to spout
out in two places. The bulk of the Ascidiae are very small, at the most a
few inches long. A few species are a foot or more in length. There are
many species of them, and they are found in every sea. As in the case
of the Acrania, we have no fossilised remains of the class, because they
have no hard and fossilisable parts. However, they must be of great
antiquity, and must go back to the primordial epoch.
The name of "Tunicates" is given to the whole class to which the
Ascidiae belong, because the body is enclosed in a thick and stiff

covering like a mantle (tunica). This mantle--sometimes soft like jelly,
sometimes as tough as leather, and sometimes as stiff as cartilage--has
a number of peculiarities. The most remarkable of them is that it
consists of a woody matter, cellulose--the same vegetal substance that
forms the stiff envelopes of the plant-cells, the substance of the wood.
The tunicates are the only class of animals that have a real cellulose or
woody coat. Sometimes the cellulose mantle is brightly coloured, at
other times colourless. Not infrequently it is set with needles or hairs,
like a cactus. Often we find a mass of foreign bodies--stone, sand,
fragments