Phylum Cephalorhyncha: Tiny Marine Worms

Etymology of Cephalorhyncha: From the Greek Kephalos for head and Rhynchos (snout or beak)

There are 237 species of Cephalorhyncha known to science (COL).

Characteristics of Cephalorhyncha:

1. Bilaterally symmetrical, and vermiform.
2. Body has more than two cell layers, tissues and organs.
3. Body cavity is a true coelom or is pseudocoelomate.
4. Body possesses a through gut with an anus.
5. Body in is in two or three sections, a prosoma and a trunk (sometime a head).
6. Possess a spiny proboscis that can be everted or held within the body.
7. Possess a cuticle that can be shed.
8. Has a nervous system with an anterior nerve ring
9. Has a ganglionated, or double nerve chord, or a brain.
10. Has no circulatory but blood corpuscles exist in the coelomic fluid.
11. Reproduction sexual and gonochoristic.
12. Has a distinct larval stage.
13. Live in marine environments.

Introduction to the Cephalorhyncha

The Cephaloryncha is a relatively new phylum created to include three older phyla; Kinorhyncha, Loricifera and Priapulida. Most Cephalorhyncans are very small, less than one millimeter long so you are unlikely to meet one in the wild.  They are all marine and mostly live in and on the soft sediment, or small particle gravel of the sea/ocean floor.  All of the Cephalorhyncha have a spiny proboscis which can be retracted into, and held inside, the head.  This proboscis is everted unfolded (pushed/rolled out) so that the spines on it can be used to gather food.

The Cephalorhyncha are considered to be an ancient lineage, because, while there are no fossil Kinorhycha nor Loricifera, there are many fossil Priapulids in the Burgess Shale, indicating that they were alive around 545 to 525 million years ago.   Taxonomically the Cephalorhyncha are a sister group to the Arthropods and the Annelids.

For more detailed information on the three classes of the Cephalorhyncha follow the links below.

Taxonomy of the Cephalorhyncha

Phylum Cephaloryncha

• Class Kinorhyncha
• Class Loricifera
• Class Priapulida

Image Credits:-  Cover image by Roberto Danovaro et. al.; license CC BY 2.0