Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 391 organs, which give rise to the anal 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 oesophageal ring, which is also provided with ganglion cells. In older stages, after the disappeai-ance of the segments, the ciliary apparatu
Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 391 organs, which give rise to the anal 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 oesophageal ring, which is also provided with ganglion cells. In older stages, after the disappeai-ance of the segments, the ciliary apparatus begins to degenerate and finally vanishes; after which two strong hooked setae make their appear- ance at the sides of the nerve cord not far from the mouth, and two circles of shorter setae are formed at the hind end of the body (fig. 316). The prseoral 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 oesophageal commissures lie in the pro- boscis, and meet in front without any cerebral enlargement. Anteriorly and on the ventral side are two setse for attachment, and on the poste- rior end of the body there are sometimes circles of setffi. The anus is terminal. Echiurm Pal- lasii Guerin (^Gacrtneri Quatref., St. Vaast), coast of Belgium and England. Thalasscma f/iffas M. Miill., Italian coast. Bonellia riridis Rolando. Mediterranean. The males are small and rudimentary, and resemble Planarians. They live in the efferent ducts of the female generative organs. Order 2.—Ach.«ta=Sipunculoidea. Gephyrea with terminal mouth, dorsally placed anus, and without setce. The ante- rior recjion of the body is retractile. The Sipuncidoidea differ from the chsetiferous Gephyrea in their entire want of all traces of metameric segmentation, in the degeneration of the prjeoral lobe and in the position of the mouth and anus
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