The Development of the Brachiopod Crania (Neocrania) anomala (O. F. Müller)and its Phylogenetic Significance

Abstract

AbstractThe freely spawned eggs of Crania go through radial cleavage, embolic gastrulation, and the posteroventral part of the archenteron forms mesoderm through modified enterocoely. The blastopore closes in the posterior end of the larva. The ciliated, lecithotrophic larva has four pairs of coelomic pouches and three pairs of dorsal setal bundles. At metamorphosis, the larva curls ventrally by contraction of a pair of midventral muscles, which are extensions of the first pair of coelomic sacs; the larva attaches by the epithelium just behind the closed blastopore. The brachial valve is secreted by the middle part of the dorsal epithelium and the pedicle valve is secreted by the attachment epithelium. The second pair of coelomic sacs develop small attachment areas at the edge of the dorsal valve and become the lophophore coelom (mesocoel); the third pair of coelomic sacs become the body coelom (metacoel) with the adductor muscles. The posterior position of the closing blastopore is characteristic of deuterostomes. The ventral curving of the settling larva and the formation of both valves from dorsal epithelial areas indicate that the brachiopods have a very short ventral side as opposed to the phoronids. It is concluded that both groups have originated from a creeping ancestor with a straight gut.