Cephalodiscida is one of two orders in the class Pterobranchia, which are small, worm-shaped animals. Members belong to the hemichordates

Hemichordata

Hemichordata is a phylum of marine deuterostome animals, generally considered the sister group of the echinoderms. They date back to the Lower or Middle Cambrian and include two main classes: Enteropneusta , and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a...

. Species in this order are sessile

Sessility (zoology)

In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...

, living in clear water and
secrete

Secretion

Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...

 tubes on the ocean floor.

Taxonomy

The order is a small one, with only two known families, each containing a single genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

.

Order Cephalodiscida

• Family Atubaridae
  • Genus Atubaria
    • Species Atubaria heterolopha Sato, 1935
• Family Cephalodiscidae
  • Genus Cephalodiscus
    • Cephalodiscus agglutinans Harmer & Ridewood, 1914
    • Cephalodiscus atlanticus Bayer, 1962
    • Cephalodiscus australiensis author unknown
    • Cephalodiscus calciformis Emig, 1977
    • Cephalodiscus densus Andersson 1907
    • Cephalodiscus dodecalophus McIntosh 1882
    • Cephalodiscus evansi Ridewood
    • Cephalodiscus fumosus John, 1932
    • Cephalodiscus gilchristi Ridewood, 1908
    • Cephalodiscus gracilis Harmer 1905
    • Cephalodiscus graptolitoides Dilly 1993
    • Cephalodiscus hodgsoni Ridewood, 1907
    • Cephalodiscus indicus Schepotieff 1909
    • Cephalodiscus kempi John, 1932
    • Cephalodiscus levinsoni Harmer
    • Cephalodiscus nigrescens Lankester, 1905
    • Cephalodiscus sibogae Harmer 1905
    • Cephalodiscus solidus Andersson, 1907