each of the spiral valves of the
seed-vessel turns rather more than twice round the circumference, the
whole together making between ten and eleven rings. The number of these
rings differs greatly in different species, but in the same appears to
be very constant.

The stems of Charae occur fossil in the Scotch marl in great abundance.
In some species, as in _Chara hispida_, the plant when living contains
so much carbonate of lime in its vegetable organization, independently
of calcareous incrustation, that it effervesces strongly with acids when
dry. The stems of _Chara hispida_ are longitudinally striated, with a
tendency to be spiral. These striae, as appears to be the case with all
Charae, turn always like the worm of a screw from right to left, while
those of the seed-vessel wind round in a contrary direction. A cross
section of the stem exhibits a curious structure, for it is composed of
a large tube surrounded by smaller tubes (fig. 103., _b_, _c_) as is
seen in some extinct as well as recent species. In the stems of several
species, however, there is only a single tube.[1099]

[Illustration: Fig. 103.

Stem and branches of Chara hispida.

_a_, Stem and branches of the natural size.

_b_, Section of the stem magnified.

_c_, Showing the central tube surrounded by two rings of smaller tubes.

]

The valves of a small animal called cypris (_C. ornata?_ Lam.) occur
completely fossilized, like the stems of Charae, in the Scotch travertin
above mentioned. The same cypris inhabits the lakes and ponds of
England, where, together with many other species, it is not uncommon.
Although extremely minute, they are visible to the naked eye, and may be
observed in great numbers, swimming swiftly through the waters of our
stagnant pools and ditches. The antennae, at the end of which are fine
pencils of hair, are the principal organs for swimming, and are moved
with great rapidity. The animal resides within two small valves, not
unlike those of a bivalve shell, and moults its integuments annually,
which the conchiferous mollusks do not. The cast-off shells, resembling
thin scales, and occurring in countless myriads in many ancient
freshwater marls, impart to them a divisional structure, like that so
frequently derived from plates of mica.

[Illustration: Fig. 104.

_Cypris unifasciata_, a living species, greatly
magnified.

_a_, Upper part. _b_, Side view of the same.

]

[Illustration: Fig. 105