Cephalodiscida
Cephalodiscida is one of two orders in the class Pterobranchia, which are small, worm-shaped animals. Members belong to the hemichordates. Species in this order are sessile, living in clear water and secrete tubes on the ocean floor. Taxonomy. The order is a small one, with only two known families, each containing a single genus. The validity of the family Atubaridae, who has only a sole member, is regarded as questionable. References. Harmer. S. F. 1905. The Pterobranchia ol the Siboga-Expedition. Siboga Exped. Monogr. 26. 1-131