called
_Brachiopoda_, a class which we may, at least provisionally, consider as
belonging to the mollusca. These _Brachiopods_ are also called
"Lamp-shells," from a certain resemblance which many of them show to the
form of a classical lamp. They are interesting, because in very ancient
times they seem to have held that place in the world's animal population
which is now held by the Lamellibranchs, by which, as they died out,
they have been gradually replaced till but comparatively few forms
survive. Some of these, however, are of great antiquity, and one of
them, _Lingula_, is, though still living, one of the most ancient of all
known animals.

We may next pass to a small sub-kingdom which includes the curious and
inert animals before referred to[16] as "Sea-squirts," Tunicaries or
Ascidians, and which constitute the sub-kingdom TUNICATA. These are
marine organisms of very simple but very peculiar structure which
sometimes grow up in compound aggregations. Certain forms (_e.g._,
_Pyrosoma_) are luminous at night and may be seen swimming about in the
ocean like so many red-hot urn-heaters. As we shall hereafter see, the
reproductive processes and the earlier stages of existence of these
creatures possess much interest, and have afforded strong grounds for
regarding them, in spite of their lowly organization, as very close
allies of the highest animals or _Vertebrata_.

Returning now to the "lobster" (lately mentioned as one of those animals
commonly called "shell-fish") we may regard it as an example of what is
by far the most numerous of all the sub-kingdoms of animals. This
sub-kingdom is made up of animals with jointed feet or "Arthropods," and
the ARTHROPODA are subdivided into four classes--1, _Crustacea_; 2,
_Myriapoda_; 3, _Arachnida_; and 4, _Insecta_; and it is to the first of
these four classes that the lobster belongs.

The class _Crustacea_ contains, besides the lobster (and its near
allies, hermit-crabs, prawns, shrimps, and cray-fish), all crabs,
including those very quaint-looking animals (now so often seen in our
living collections), the king-crabs (_Limulus_), and a variety of more
or less strangely different forms such as the following:--

Certain Crustaceans, of the group called _Ostracods_, have the hard
outer coat of their body so peculiarly modified that they have quite the
appearance of Lamellibranch Mollusks, and this resemblance is even more
than skin deep, as we shall see later.

Some of