Anatomy of the cephalochordate Amphioxus.

http://en.wikipedia.org/wiki/Chordate

1 = bulge in spinal cord ("brain")

2 = notochord

3 = dorsal nerve cord

4 = post-anal tail

5 = anus

6 = digestive canal

7 = circulatory system

8 = atriopore

9 = space above pharynx

10 = pharyngeal slit (gill)

11 = pharynx

12 = vestibule

13 = oral cirri

14 = mouth opening

15 = gonads (ovary / testicle)

16 = light sensor

17 = nerves

18 = metapleural fold

19 = hepatic caecum (liver-like sack)

Anatomy of the cephalochordate Amphioxus. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.