2SO ' 



CCELHELMINTHES 



or spindle-shaped, circular in section. The mouth, at the extreme 

 anterior end, is either surrounded by a circle of tentacles (fig. 266), 

 retracted together with the anterior end of the body by internal muscles, 

 or is overhung by a dorsal preoral lobe or proboscis which may be several 

 times the length of the body and forked at its tip (fig. 268). 



Internal segmentation is also lost, septa being entirely lacking. The 

 nephridia are reduced in number, at most but three pairs being present, 

 and in some but a single unpaired organ. They are the sexual ducts ; 

 the duetiferi have special excretory organs (fig. 268, g) covered with 

 branching canals opening to the body cavity by nephrostomes and 



,4 



B 



FIG. 268. Bonellia viridis. A, female (after Huxley); B, male (after Spengel). c, 

 cloaca; d, rudimentary intestine; g, excretory organ ; i, intestine; m, muscles supporting 

 intestine; 5, balls of spermatozoa in B, in A, proboscis (preoral lobe); u, single segmental 

 organ, functioning as oviduct ;vd, nephridium with ciliated funnel serving as vas deferens. 



emptying into the intestine. These resemble somewhat the branchial 

 trees of the holothurians (infra), and hence the gephyrasa were formerly 

 supposed to bridge the gap between holothurians and annelids, whence the 

 name (ye'^vpa, bridge). The vascular and nervous systems are more 

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

 around the digestive tract and a dorsal and usually a ventral longitudinal 

 trunk; the nervous system of a brain, cesophageal collar, and ventral cord, 



