The separation and growth of the
spiral arms seem to be due to the
widening or expansion of the me-
dian lobe or tentacle, on each side
This is very


apparent in the young Discinisea described by Miiller,” and
the Glottidia described by Brooks. °®
The brachidium in Zygospira passes through a series of


DEVELOPMENT OF THE BRACHIOPODA 283


changes which have been described in detail elsewhere.‘
These metamorphoses are of great assistance in understand-
ing the development and comparative morphology of this
feature in other groups of the Spiriferacea. The earliest
stage observed (figure 125, a) has the form of a simple tere-
bratuloid loop, which, from its resemblance to Centronella,
was called the centronelliform stage. Since approximately
this form of brachidium is also characteristic of the young
of recent terebratuloids, it may be taken in Zygospira as
indicative of the trocholophus stage of brachial development.
With this as a starting-poimt for comparison, the further
correlation of the succeeding stages is very simple.
The first resorption of the end of the loop in Zygospira
produced a schizolophus condition, and further resorption
carried the brachidium to a stage closely resembling Dielasma
(figure 125, 6). The dielasmatiform stage has already been
explained as due to the requirements of space for the growth
of the coiled brachia. Next, the initial calcification of
the spiral arms resulted in the extension of the descending
branches beyond the jugum (figure 125, ¢c), and, lastly, com-
plete calcification manifests the spirolophus structure and
produced the characteristic brachidium of the Spiriferacea.
