428 
MOLLUSC OIDEA. 
shell. The differences in the forms of this framework are used for distinguishing 
the families and other subdivisions of the Brachiopoda. Its form and extent can 
be made out in well-preserved fossil specimens. These two spirally coiled append¬ 
ages are the “ arms,” from which the class takes its name, and deserve a little closer 
description. The illustration below represents the under surface of a valve of the 
serpent-headed Terebratula, near the back of which the mouth opens ( 0 ). The 
arms rise on each side of the mouth, run forward, bend 
back again, to end in an elegant coil. We may compare 
them to the crown of tentacles and other organs associ¬ 
ated with the mouth of many worms, and of the moss- 
animals, only here they are stiffened by a calcareous 
skeleton. As can be gathered from their rigid calcareous 
frame, they are capable of only slight movement, and 
even their fringes are more or less stiff. As to the 
functions of these arms, inasmuch as they are traversed 
by canals, and covered with cilia, we are justified in 
assuming them to be gills. The animals feed on fine 
particles brought to the mouth by the streams of water set up by the cilia on these 
gills. In only one genus (Rltynconella), belonging to the family of th eRhynconellidce, 
do the arms themselves project beyond the shell to seize prey. The alimentary 
canal is short, and ends blindly at x. The body of the animal within the shell is 
enveloped in two mantle-folds closely applied to the shell; the latter being formed 
by a secretion from the outer surface of this mantle. Very simple reproductive 
organs lie in vessel-like widenings of these mantle-folds. The sexes are separate, 
and may sometimes be recognised by differences in the form of the shell. A pair 
of membranous funnels, internally ciliated, serve as ducts for the genital products; 
the free ends of these funnels open into the body-cavity, and conduct the reproductive 
elements outwards. The resemblance of these funnels to the nephridia of worms, 
would hardly be enough to establish a relationship between the Brachiopods and the 
worms. Their relationships rest rather upon the developmental history of the 
Brachiopods and the transformations they undergo. The first stages in the develop¬ 
ment of Tliecidium are briefly as follows. The developing eggs enter a pouch 
formed by the lower mantle-fold, into which also the two nearest arm-fringes sink. 
These latter become thicker, their ends swelling to form a pair of pads, round 
which the eggs group, and to which each embryo is attached by means of a short 
stalk. The embryo soon resembles a short thick annelid. The upper process 
from its neck is the stalk by means of which the embryo is attached to the arm- 
fringe. The small anterior section resembles a head, and carries four eye-spots 
and a depression, the future mouth. There are two thicker middle segments, 
followed by a fourth smaller segment, all covered with cilia. I 11 the later 
development the most posterior part is used for attachment, the head and collar¬ 
like ring sink between the upward growing portions of the following ring. 
These upward growths increase and form the two mantle-folds. These, as above 
stated, secrete the shell. The illustration (6), shows the young Thecidium with¬ 
drawn into itself, having given up the free-swimming life it led after breaking 
away from its parent. 
back valve of Terebratulina, 
SHOWING THE ARMS. 
