Article: The origin of the loop in articulate brachiopods
The lophophore of living articulate brachiopods develops from a pair of generative zones which are first located antero-medianly of the mouth, and are each responsible for the proliferation initially of a single set of filaments, constituting the trocholophe, and subsequently of a paired set of filaments. Brachidial supports are secreted within sheaths of outer epithelium and grow independently of the lophophore, but those developed in shells about 1 mm. in length invariably support a simple, subcircular lophophore; and since the earliest known loop, that of the spiriferoid Protozyga, was fully developed in sexually mature shells of that size, it probably appeared coenogenetically in a neotenous stock. The modal size of the earliest terebratuloid loops compares with that of the primary coils posterior of the jugum in contemporary spiriferoids, so that the terebratuloids could have been derived neotenously out of the spiriferoids. The loop of the pentameroid Enantiosphen is only superficially comparable with the centronellaceid one, but that found in the dalmanellaceid Tropidoleptus is a remarkable homeomorph of the terebratellaceid loop and probably differed only in the style of lophophore it supported.