DEVELOPMENT OF BRACHIOPODS. 151 
eases much in size so as to partially enclose the cephalic 
gment, asat C. The form of the Brachiopod is then soon 
tained, as seen in D, in which the head (c) is seen project- 
g from the two valves of the shell (¢h), the larger being 
e ventral plate. 
The hinge margin is broad and slightly rounded when 
oked at from above ; a side view, however, presents a wide 
d flattened area, as is shown in some species of Spirifer, 
id the embryo for a long time takes the position that the 
virifer must have assumed (Morse). Before the folds have 
osed over the head, four bundles of bristles appear ; these 
istles are delicately barbed like those of larval worms. 
he arms, or cirri, now bud out as two prominences, one on 
ch side of the mouth. Then as the embryo advances in 
owth the outlines remind one of a Leptena, an ancient 
mus of Brachiopods, and in a later stage the form becomes 
lite unlike any adult Brachiopod known. 
The deciduous bristles are then discarded, and the perma- 
mt ones make their appearance, two pairs of arms arise, 
id now the shell in ‘its general contour recalls Siphono- 
eta, placed in the family Discinide by Davidson, a genus 
t occurring above the Silurian.” No eye-spots could be 
en in Terebratulina, though in the young Thecidium they 
are observed by Lacaze-Duthiers. The young Terebratu. 
ia differs from Discina of the same age in being sedentary, 
hile, as observed by Fritz Miller, the latter ‘‘swims freely 
the water some time after the dorsal and ventral plates, 
rri, mouth, oesophagus and stomach have made their ap- 
rarance.” Discina also differs from Terebratulina in hav- 
g a long and extensible esophagus and head bearing a 
own of eight cirri or tentacles. Regarding the relations 
‘the Brachiopods with the Polyzoa, Morse suggests that 
ere is some likeness between the young Brachiopod and 
e free larva of Pedicellina. Fig. 103, B, represents the 
arebratulina when in its form it recalls Megerlia or Argi- 
vr. C represents a later Lingula-like stage. ‘It also 
ggests,” says Morse, ‘‘in its movements, the nervously 
ting Pedicellina. In this and the several succeeding 
ages, the mouth points directly backward (forward of 
