ypify, in their adult condition, the larval state
of the higher forms of the group. Thus, among the amphibia, frogs have
tails in the larval or tadpole condition; but newts throughout life
remain in the larval or tailed condition. Appendicularia he considered
to be the lowest form of the Ascidians, and to typify in its adult
condition the larval stages of the higher Ascidians.

By this remarkable investigation of the structure of the group of
Ascidians, and display of the various grades of organisation, Huxley
paved the way for one of the great modern advances in knowledge. When,
later on, the idea of evolution was accepted, and zooelogists began
hunting out the pedigree of the back-boned animals, it was discovered
that Ascidians were modern representatives of an important stage in
the ancestry of vertebrate animals, and, therefore, of man himself.
There are few more interesting chapters in genealogical zooelogy than
those which reveal the relationship between Amphioxus and fish on the
one hand, and Ascidians on the other; for fish are vertebrates, and
Ascidians, on the old view, are lowly invertebrates. The details of
these relationships have been made known to us by the brilliant
investigations of several Germans, by Kowalevsky, a Russian, by the
Englishmen Ray Lankester and Willey, and by several Americans and
Frenchmen. But behind the work of all these lies the pioneer work of
Huxley, who first gathered the group of Ascidians together, and in a
series of masterly investigations described its typical structure.

Huxley's next great piece of work was embodied in a memoir published
in the _Transactions of the Royal Society_ in 1853, and which remains
to the present day a model of luminous description and far-reaching
ideas. It was a treatise on the structure of the great group of
molluscs, and displays in a striking fashion his method of handling
anatomical facts, and deducing from them the great underlying
principles of construction. The shell-fish with which he dealt
specially were those distinguished as cephalous, because, unlike
creatures such as the oyster and mussel, they had something readily
comparable with the head of vertebrates. He began by pointing out what
problems he hoped to solve. The anatomy of many of the cephalous
molluscs was known, but the relation of structures present in one to
structures present in another group had not been settled.

"It is not settled whether the back of a cuttle-fish a