like structure.
Another branch of the family then abandoned the stalk, and, spreading
its arms flat, and gradually developing in them numbers of little "feet"
(water-tubes), became the starfish. In the living Comatula we find a
star passing through the stalked stage in its early development, when it
looks like a tiny sea-lily. The sea-urchin has evolved from the star by
folding the arms into a ball. [*]

* See the section on Echinoderms, by Professor MacBride, in
the "Cambridge Natural History," I.

The Bryozoa (sea-mats, etc.) are another and lower branch of the
primitive active organisms which have adopted a sessile life. In the
shell-fish, on the other hand, the principle of armour-plating has its
greatest development. It is assuredly a long and obscure way that
leads from the ancestral type of animal we have been describing to
the headless and shapeless mussel or oyster. Such a degeneration is,
however, precisely what we should expect to find in the circumstances.
Indeed, the larva, of many of the headless Molluscs have a mouth and
eyes, and there is a very common type of larva--the trochosphere--in the
Mollusc world which approaches the earlier form of some of the
higher worms. The Molluscs, as we shall see, provide some admirable
illustrations of the process of evolution. In some of the later
fossilised specimens (Planorbis, Paludina, etc.) we can trace the animal
as it gradually passes from one species to another. The freshening of
the Caspian Sea, which was an outlying part of the Mediterranean quite
late in the geological record, seems to have evolved several new genera
of Molluscs.

Although, therefore, the remains are not preserved of those primitive
Molluscs in which we might see the protecting shell gradually
thickening, and deforming the worm-like body, we are not without
indications of the process. Two unequal branches of the early wormlike
organisms shrank into strong protective shells. The lower branch became
the Brachiopods; the more advanced branch the Molluscs. In the Mollusc
world, in turn, there are several early types developed. In the
Pelecypods (or Lamellibranchs--the mussel, oyster, etc.) the animal
retires wholly within its fortress, and degenerates. The Gastropods
(snails, etc.) compromise, and retain a certain amount of freedom, so
that they degenerate less. The highest group, the Cephalopods, "keep
their heads," in the literal sense, and we shall find them advancing
from for