age 79]

Again, the aerial mammals, the bats, resemble those flying reptiles of the
secondary epoch, the pterodactyles; not only to a certain extent in the
breast-bone and mode of supporting the flying membrane, but also in the
proportions of different parts of the spinal column and the hinder (pelvic)
limbs.

Also bivalve shell-fish (_i.e._ creatures of the mussel, cockle, and oyster
class, which receive their name from the body being protected by a double
shell, one valve of which is placed on each side) have their two shells
united by one or two powerful muscles, which pass directly across from one
shell to the other, and which are termed "adductor muscles" because by
their contraction they bring together the valves and so close the shell.

[Illustration: CYTHERIDEA TOROSA.
[An ostracod (Crustacean), externally like a bivalve shell-fish
(Lamellibranch).]

Now there are certain animals which belong to the crab and lobster class
(Crustacea)--a class constructed on an utterly different type from that on
which the bivalve shell-fish are constructed--which present a very curious
approximation to both the form and, in a certain respect, the structure of
true bivalves. Allusion is here made to certain small Crustacea--certain
phyllopods and ostracods--which have the hard outer coat of their thorax so
modified as to look wonderfully like a bivalve shell, although its {80}
nature and composition are quite different. But this is by no means
all,--not only is there this external resemblance between the thoracic
armour of the crustacean and the bivalve shell, but the two sides of the
ostracod and phyllopod thorax are connected together also by an adductor
muscle!

[Illustration: A POLYZOON WITH BIRD'S-HEAD PROCESSES.]

{81}
The pedicellariae of the echinus have been already spoken of, and the
difficulty as to their origin from minute, fortuitous, indefinite
variations has been stated. But structures essentially similar (called
avicularia, or "bird's-head processes") are developed from the surface of
the compound masses of certain of the highest of the polyp-like animals
(viz. the Polyzoa or, as they are sometimes called, the Bryozoa).

These compound animals have scattered over the surface of their bodies
minute processes, each of which is like the head of a bird, with an upper
and lower beak, the whole supported on a slender neck. The beak opens a