Hemichordata

Abstract Hemichordates consist of the two exclusively marine taxa, Pterobranchia (angelwings) and Enteropneusta (acorn worms). All hemichordates feature three body parts: the prosome, the mesosome, and the metasome. The major part of the nervous system in both hemichordate groups comprises a plexus. In acorn worms, there is a dorsal collar cord beneath the epidermis in the mesosome. This includes mainly neurites, but also some neuron somata and giant neurons. It has a fluid-filled central canal. In pterobranchs the basiepidermal plexus is ubiquitous with local differences in density, notably the brain, a more prominent ventral nerve cord in the trunk extending into the stalk, a denser area on the dorsal side of the cephalic shield just anterior of the brain, and a considerably less dense area in the dorsal part. Differentiated sensory organs have not been described in adult hemichordates, although eyes are present in tornaria larvae.