Pvn H,i I'UitlS

4 BRACHIOPODA.
It deservedly gained him the Royal medal, and is the
more praiseworthy because he never, I believe, had the
good fortune to see a living specimen. This opportunity,
however, has frequently occurred to me ; and I
will endeavour,
treatises, to present
with the aid of Mr. Hancock's and other
habits of this extraordinarv class.
a few remarks on the structure and
It has been usual to consider the valves of the shell
in Terebratula as covering the front and back of the
animal, the perforate valve being ventral, and the imperforate
valve dorsal. When the Terebratula is attached
by its peduncle the perforate valve is uppermost. But
the analogy between the Brachiopoda and the Vertebrata
is very slight. The back of a Terebratula is
really
that part which lies behind the arms and mouth, and is
close to the apex or point of attachment. Instead,
therefore, of calling the valves " ventral" and " dorsal,"
it would seem more correct to describe them as "upper"
and " lower "—the larger and deeper valve being perfo-
rate and uppermost, and the smaller and shallower valve
being imperforate and lowermost. In the Brachiopoda
the valves are articulated across the back ; in the Con-
chifera the valves are united by a ligament or cartilage
along the back. The arms occupy two-thirds of the
shell. They resemble the mainspring of a watch, and
are not capable of being protruded or unrolled. I have
never observed the cirri, with which they are clothed,
to project much beyond the edges of the shell in the
living animal. The great extent of these brachial organs
is very remarkable. In Rhynchonella psittacea the arms,
when forcibly stretched out, are said to be more than
four times the length of the shell, and to support about
3000 cirri. In Terebratula caput-serpentis the cirri
open and fold together somewhat like a butterfly-net.