392 



ANNELIDA. 



organs, which give rise to the a mil vesicles (fig. 315 a, AS). The 

 rudiments both of the cerebral ganglion and of the ventral cord are 

 derived from growths of the ectoderm, the former from the apical 

 plate, the latter as a paired thickening of the ventral ectoderm. The 

 two are connected by the cesophageal ring, which is also provided 

 with ganglion cells. In older stages, after the disappearance of the 

 segments, the ciliary apparatus begins to degenerate and finally 

 vanishes; after which two strong hooked seta? make their appear- 

 ance at the sides of the nerve cord not far from the mouth, and 



two circles of shorter seta? are formed at 

 the hind end of the body (fig. 316). 

 The pra?oral lobe of the larva becomes 

 the proboscis of the young Echiurus (fig. 

 311). 



Fam. Echiuridae. The anterior end of the 

 body above the mouth is elongated into a pro- 

 boscis, the under surface of which is grooved. The 

 long cesophageal commissures lie in the pro- 

 boscis, and meet in front without any cerebral 

 enlargement. Anteriorly and on the ventral side 

 are two setee for attachment, and on the poste- 

 rior end of the body there are sometimes circles 

 of setre. The anus is terminal. Ei-li'nirux Pal- 

 Ittxii Gnerin (Gtirrtnert Quatref., St. Vaast), 

 coast of Belgium and England. Thalassema 

 (/if/nx M. Miill.. Italian coast. linn-cU'ni cii-idix 

 Rolando. Mediterranean. The males are small 

 and rudimentary, and resemble Plaiiarians. 

 They live in the efferent ducts of the female 

 generative organs. 



BK 



Order 2. ACHJETA= SIPUNCULOIDEA. 



Gephyrea with terminal mouth, dorsally 

 placed anus, and without setce. The ante- 

 rior region of the body is retractile. 



The Sipunculoidea differ from the 

 chsetiferous Gepliyrea in their entire want 

 of all traces of nietameric segmentation^ 

 in the degeneration of the praeoral lobe 

 and in the position of the mouth and anus. 

 The elongated body is destitute of a projecting prseoral lobe, so that 

 the mouth, which is frequently surrounded by a circle of tentacles, 

 comes to be placed at the anterior end of the body. On the other 

 hand, the anus is moved far forward on the dorsal surface (fig. 317). 



FIG. 316. Older Echiurus larva 

 seen from the side. The head 

 kidney is atrophied. O, 

 mouth ; M, stomach ; A, anus ; 

 -B-Zr, circles of set* ; SC, ceso- 

 phageal commissure ; AS, 

 anal vesicles ; G- , cerebral 

 ganglion, developed from the 

 apical plate; 7"V/, ventral 

 nerve cord ; H, ventral hooks. 



