CHAPTEE XI 



GEPHYREA 



GetihvTpa -f ■^''h^^ta — Phymosoma, Sipimculus, Fhascolion. 

 r y^ 1^ Chaetifera — Bonellia, Echiurus, Thalassema. 



Chaeacteeistics. — Subcylindrical marine animals, with very 

 slight indications of segmentation. The anterior part of the 

 body is either retractile, forming an Introvert, or it hears an 

 extensile Prostomiwm. Setae or chitinoid hoohs 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 ly 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 The introvert is withdrawn 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 hoohs. The mouth is anterior and terminal. 



IT. 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. 



