///. ANNELIDA: GEPHYR^A. 317 



dorsal spatulate preoral lobe or * proboscis ' which may be several 

 times the length of the body and forked at its tip (fig. 289). 



Internal segmentation is also lost, septa being entirely lacking. 

 The nephridia are also reduced in number, at most but three pairs 

 being present, and in some but a single unpaired organ. They 

 are sexual ducts, and in the Chaetiferi there are special excretory 

 organs (fig. 289, g] covered with branching canals opening to the 

 body cavity by nephrostomes and emptying into the intestine. These 

 resemble somewhat the branchial trees of the holothurians (infra), 

 and hence these animals were formerly supposed to bridge the gap 

 between holothurians and annelids, whence the name (yetyvpa, 

 bridge) Gephyrsea. The vascular and nervous systems are more 

 like those of other annelids. The vascular system consists of a 

 dorsal and usually a ventral longitudinal trunk; the nervous 

 system of a brain, oesophageal collar, and ventral cord, the latter 

 without division into ganglia. The relations of the Gephyrsea 

 to the Chaetopoda are shown by the development. In some 

 (Chaetiferi) there is a trochophore (fig. 288) from which the worm 

 arises, as in the Chaetopoda, by growth at the hinder end; this at 

 first has a segmented coalom and nervous system, the metamerism 

 being lost later. 



Order I. Chaetiferi (Armata, Echiuroidea). 



With spatulate preoral lobe, often forked at the tip; at least a pair of 

 ventral setae; a trochophore in development. Ecliiurus pallasii* in our 

 northern waters, Thalassema* farther south. In Bonellia there is a 

 marked sexual dimorphism (fig. 289). The female is 2 to 3 inches long and 

 has a proboscis 8 to 12 inches long. The male is only 1 mm. long, totally 

 different in appearance, and lives parasitically in the oesophagus of the 

 female (fig. 289, B). 



Order II. Inermes (Achaeta, Sipunculoidea). 



Distinguished by lack of chsetse, the mouth surrounded by tentacles, 

 and the dorsal and anterior position of the anus. The larva is a modified 

 trochophore without preoral ciliated band and without segmentation; only 

 two, sometimes but one, nephridia. Phascalosoma* common on our 

 shores. Phascolion strombi * builds tubes in deserted snail shells. Sipun- 

 culus.* 



Order III. Priapuloidea. 



No tentacles, mouth with chitinous teeth, terminal anus, no nephridia; 

 two protonephridia united with the sexual organs and opening either side 

 of the vent. Development unknown. P?*iapulus, Halicryptus. 



