Cephalochordata
1. Dorsal fin and fin ray. 2. Dorsal nerve cord. 3. Notochord. 4. Rostrum.
5. Vestibule. 6. Wheel organ. 7. Oral cirri. 8. Velum. 9. Gill bar
Another diagram showing internal structures.
1. brain-like blister 2. notochord 3. dorsal nerve cord 4. post-anal tail 5. anus 6. food canal 7. blood system 8. abdominal porus 9. overpharynx lacuna 10. gill's slit 11. pharynx 12. mouth lacuna 13. mimosa 14. mouth gap 15. gonads (ovary/testicle) 16. light sensor 17. nerves 18. abdominal ply 19. hepatic caecum
Water is taken in through the mouth, drawn in by the beating of cilia located on the wheel organ, a set of ridges lying inside the mouth. The water is first filtered by the oral cirri, slender projections that surround the opening of the mouth, clearly visible on the photograph above. It then passes through the gill slits. These gill slits are enclosed by folds of the body wall, the metapleural folds, to form a body cavity known as the atrium. Food particles in the water are trapped by mucus, while water passes through the slits and out of the atrium through the atriopore, located towards the posterior end.
The rest of the digestive system is fairly simple: a pouch or hepatic caecum secretes digestive enzymes, and actual digestion takes place in a specialized part of the intestine known as the iliocolonic ring. Cephalochordates also have a well-developed circulatory system and a simple excretory system composed of paired nephridia. The sexes are separate, and both males and females have multiple paired gonads. Eggs are fertilized externally, and develop into free-swimming, fishlike larvae.
Unlike tunicates, the lancelets exhibit these features as adults and as larvae. Lancelets also show bilateral symmetry, and are much longer than they are wide. They have blade like structures on the outside of their body and have segmented muscles that can bend, allowing the animal to slowly swim. However, lancelets live in marine sands and feed with the majority of their body submerged in the sand but with their head sticking out. The gill apparatus traps food particles in the water, just like the process that occurs in tunicates. It is believed that lancelets are the closest living relatives of vertebrates, and they shared a common ancestor that was stationary and probably resembled an adult tunicate. This subphylum is very small, consisting of only about 20 species.
This song dates back to 1920 or so. It has been attributed to Dr. Sewell H. Hopkins of the Zoology Department of Texas A&M, but recent information sent to me suggests that he did not write the song, but gained fame for declaiming it in class as a poem. Hopkins claimed to have learned it in the 1920s at Woods Hole
It's a long way from amphioxus
It's a long way to us…
It's a long way from amphioxus
To the meanest human cuss.
It's good-bye, fins and gill slits,
Hello, lungs and hair!
It's a long, long way from amphioxus,
But we all came from there!
A fish-like thing appeared among the annelids one day;
It hadn't any parapods or setae to display.
It hadn't any eyes or jaws, or ventral nervous chord,
But it had a lot of gill slits and it had a notochord.
(chorus)
It wasn't much to look at, and it scarce knew how to swim.
And Nereis was very sure it hadn't come from him.
The molluscs wouldn't own it, and the arthropods got sore,
So the poor thing had to burrow in the sand along the shore.
He burrowed in the sand before a crab could nip his tail.
He said "Gill slits and myotomes are all to no avail.
I've grown some metapleural folds, and sport an oral hood.
And all these fine new characters don't do me any good!"
(chorus)
He sulked a while down in the sand without a bit of pep.
Then he stiffened up his notochord and said "I'll beat 'em yet!
Let 'em laugh and show their ignorance; I don't mind their jeers!
Just wait until they see me in a hundred million years!"
"My notochord shall turn into a chain of vertebrae;
As fins, my metapleural folds will agitate the sea.
My tiny dorsal nervous chord shall be a mighty brain
And the vertebrates will dominate the animal domain!"
(chorus)
Fossils known from the Cambrian.
http://www.ucmp.berkeley.edu/chordata/cephalo.html