CHAPTER, Xi 
GEPHYREA 
{ Achaeta—Phymosoma, Sipuneulus, Phascolion. 
| Chaetifera—Bonellia, Echiurus, Thalassema. 
Gephyrea 
CHARACTERISTICS.—Subeylindrical marine animals, with very 
slight indications of segmentation. The anterior part of the 
body is either retractile, forming an Introvert, or it bears an 
extensile Prostomium.  Setae or chitinoid hooks usually 
present. The nervous system consists of a circum-oesophageal 
ring and a ventral non-ganglionated cord. No special re- 
spiratory or locomotor organs exist. A closed vascular system 
is present, and the coelom is spacious. Nephridia are present, 
and serve as a rule as ducts for the exit of the reproductive 
cells, as well as functioning as excretory organs. 
The Gephyrea are divided into two sub-classes : 
I. The Achaeta, characterised by the anterior end of the 
body being retractile. The introvert is withdrawn 
into the body by special retractor muscles in the same 
way as the proboscis of a Nemertine. No setae are 
found, but the introvert is usually armed by rows of 
chitinoid hooks. The mouth is anterior and terminal. 
II. The Chaetifera have a segmented larva, but the segmenta- 
tion is lost in the adult. There is a long extensile 
prostomium which is easily broken off. A pair of 
ventral setae are found. Anus terminal; special 
branching organs open on the one hand into the coelom, 
and on the other into the rectum, they possibly function 
as nephridia. One to eight anterior nephridia serve 
as genital ducts. 
