l well
developed. No stalk has been seen by Simroth or Fritz Muller, but in
other respects the larva resembles the stages in the development of
_Megathyris_ and _Terebratulina_ which immediately precede fixation.
The cirri or tentacles, of which three or four pairs are present, are
capable of being protruded, and the minute larva swims by means of the
ciliary action they produce. It can retract the tentacles, shut its
shell, and sink to the bottom.

[FIG. 30.--Stages in the fixing and metamorphosis of _Terebratulina_.
Highly magnified. (From Morse.)

A, Larva (neo-embryo) just come to rest.
B, C, D, Stages showing the turning forward of the second or mantle
segment.
E, Completion of this.
F, Young Brachiopod.
1, 2, 3, The first, second and third segments.]

C.E.E. Beecher (_Amer. Jour. Sci._ ser. 3, xli. and xliv.) has
classified with appropriate names the various stages through which
Brachiopod larvae pass. The last stage, that in which the folds of the
second segment are already reflected over the first, he calls the
Typembryo. Either before or just after turning, the mantle develops a
larval shell termed the protegulum, and when this is completed the
larva is termed the Phylembryo. By this time the eyes have
disappeared, the four bundles of chaetae have dropped off, and the
lophophore has begun to appear as an outgrowth of the dorsal mantle
lobe. The protegulum has been found in members of almost all the
families of Brachiopod, and it is thought to occur throughout the
group. It resembles the shell of the Cambrian genus _Iphidea
[Paterina]_, and the Phylembryo is frequently referred to as the
_Paterina_ stage. In some orders the Phylembryo is succeeded by an
_Obolella_ stage with a nearly circular outline, but this is not
universal. The larva now assumes specific characters and is
practically adult.

[FIG. 31.--Shell of larval Brachiopod. Phylembryo stage. (From
Simroth.) 1, Protegulum; 2, permanent shell.]

_Classification_.--Beecher's division of the Brachiopoda into four
orders is based largely on the character of the aperture through which
the stalk or pedicle leaves the shell. To appreciate his diagnoses it
is necessary to understand certain terms, which unfortunately are not
used in the same sense by all authors. The triangular pedicle-opening
seen in _Orthis_, &c., has been named by James Hall and J.M. Clarke