common oyster which
annually disperses about 60,000,000 eggs. But where the risk of
destruction is slight, the eggs are large and produce young differing
from the parent only in size, as in the case of the pigeon-like eggs of
_Bulimus_.

_Crustaceans._--Among the higher Crustacea, as a rule, the eggs are
carried by the female, attached to special appendages on the under side
of the body. But in some--Squillas--they are deposited in burrows.
Generally they are relatively small so that the young which emerge
therefrom differ markedly in appearance from the parents, but in
deep-sea and freshwater species the eggs are large, when the young, on
emerging, differ but little from the adults in appearance.

_Insects, &c._--The eggs of insects though minute, are also remarkable
for the great variety of form which they present, while they are
frequently objects of great beauty owing to the sculptured markings of
the shell. They are generally laid in clusters, either on the ground, on
the leaves of plants, or in the water. Some of the gnats (_Culex_) lay
them on the water. Cylindrical in shape they are packed closely
together, set on end, the whole mass forming a kind of floating raft.
Frequently, as in the case of the stick and leaf insect, the eggs are
enclosed in capsules of very elaborate shapes and highly ornamented.

As to the rest of the Invertebrata--above the Protozoa the eggs are laid
in water, or in damp places. In the former case they are as a rule
small, and give rise to larvae; while eggs hatched on land are sometimes
enclosed in capsules, "cocoons," as in the case of the earthworm, where
this capsule is filled with a milky white fluid, of a highly nutritious
character, on which the embryos feed.

Among some invertebrates two different kinds of eggs are laid by the
same individual. The water-flea, _Daphnia_ (a crustacean), lays two
kinds of eggs known as "summer" and "winter" eggs. The summer eggs are
carried by the female in a "brood-pouch" on the back. The "winter" eggs,
produced at the approach of winter, differ markedly in appearance from
the summer eggs, being larger, darker in colour, thicker shelled, and
enclosed in a capsule formed from the shell or carapace, of the parent's
body. "Winter eggs," however, may be produced in the height of summer.
While the "summer eggs" are unfertilized, the winter eggs are fertilized
by the male, and possess the remarkable power of lying dormant for
months or even years