When we approach this task, we find an auxiliary of the utmost
importance in the comparative anatomy and embryology of two lower
animal-forms. One of these animals is the lancelet (Amphioxus), the
other the sea-squirt (Ascidia). Both of these animals are very
instructive. Both are at the border between the two chief divisions of
the animal kingdom--the vertebrates and invertebrates. The vertebrates
comprise the already mentioned classes, from the Amphioxus to man
(acrania, lampreys, fishes, dipneusts, amphibia, reptiles, birds, and
mammals). Following the example of Lamarck, it is usual to put all the
other animals together under the head of invertebrates. But, as I have
often mentioned already, the group is composed of a number of very
different stems. Of these we have no interest just now in the
echinoderms, molluscs, and articulates, as they are independent
branches of the animal-tree, and have nothing to do with the
vertebrates. On the other hand, we are greatly concerned with a very
interesting group that has only recently been carefully studied, and
that has a most important relation to the ancestral tree of the
vertebrates. This is the stem of the Tunicates. One member of this
group, the sea-squirt, very closely approaches the lowest vertebrate,
the Amphioxus, in its essential internal structure and embryonic
development. Until 1866 no one had any idea of the close connection of
these apparently very different 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