Achæta (SIPUNCULOIDEA or INERMES)

Gephyrean worms with terminal mouth, dorsal anus, no setæ, and with the anterior region of the body retractile. There is no proboscis, but sometimes ( Sipunculus ) a circle of tentacles about the mouth, and all traces of metamerism is lost. The Priapulidæ have no tentacles and the intestine is straight (coiled in Sipunculidæ ), and there are pharyngeal teeth. tions might be constructed which would give achromatic (colorle ) images. The principle is now applied to nearly all optical instruments.