advanced than in any of the others. Yet the characteristic connection
and arrangement of all the organs is just the same as in the other
vertebrates. All these, moreover, pass, during their embryonic
development, through a stage in which their whole organisation is no
higher than that of the Amphioxus, but is substantially identical with
it.

(FIGURE 2.219. Transverse section of the head of the Amphioxus (at the
limit of the first and second third of the body). (From Boveri) a
aorta (here double), b atrium, c chorda, co umlaut coeloma
(body-cavity), e endostyl (hypobranchial groove), g gonads (ovaries),
kb gill-arches, kd 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