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1 = bulge in spinal cord ("brain")
2 = notochord
3 = dorsal nerve cord
4 = postanal tail
5 = anus
6 = digestive canal
7 = circulatory system
8 = atriopore
9 = space above pharynx
10 = pharyngeal slit
11 = pharynx
12 = vestibule
13 = oral cirri
14 = mouth
15 = gonads
16 = photo sensor
17 = nerves
18 = metapleural fold
19 = hepatic caecum
