such as _Beroee_
and _Cydippe_, which are balls of glassy transparency displaying
iridescent hues as they move rapidly through the water by means of their
peculiar locomotive organs.

Other coelenterates, of the same essential type but of simpler
structure, form the class _Hydrozoa_. Amongst these may be mentioned the
little _Hydra_ of our ponds, which will often come before us in our
survey of animal life. Some compound forms of Hydrozoa simulate the
compound Actinozoa; such are the calcareous millipores, and those with a
softer structure, called "corallines," such as _Eudendrium_ and many
others. The Portuguese man-of-war (_Physalia_) and the various forms of
jelly-fish (_Medusae_) all belong to the _Hydrozoa_, as also does a very
curious and very elementary form, to which the name _Tetraplatia_ has
been given.

Next we come to the group of sponges, SPONGIDA, some of which--as the
now well-known _Euplectella_--are of marvellous beauty and delicacy of
structure; while others, as the sponge of commerce, are of much greater
simplicity of form. Simplest of all the sponges is the sponge called
_Ascetta Primordialis_. Some sponges have a horny, some a calcareous,
and some a siliceous skeleton, and (strange as it may appear) some have
a habit of boring into shells, and living in the excavations they make.

An animal recently discovered, _Dicyema_, may at this initial stage of
our inquiry be left with its place and affinities undetermined. It is a
minute worm-like creature of most exceptionally simple structure, which
lives parasitically within cuttle-fishes.

We now pass to animals (if so they are really to be considered) which
are the lowest and simplest of all, and which are mostly microscopic in
size, and may be grouped together under the term HYPOZOA, or under the
generally employed name _Protozoa_. With very few exceptions these
animals are aquatic, and if terrestrial they are found in damp
localities. Some are marine, others are fresh-water organisms.

The highest of the group are the animalcules, which are named
_Infusoria_, most of which are freely swimming organisms, though a
certain number of them live fixed to some supporting body.

Another group of _Hypozoa_ is that termed _Gregarinida_, a group made up
of very lowly parasites, such as are often found tenanting the
intestines of insects as well as those of higher animals. Finally, we
have the group of _Rhizopoda_, animals which have the faculty of
project