Cephalochordata includes two genera, 1. Asymmetron and 2. Branchiostoma (Amphioxus). Cephalochordates are small fish like animals which show Chordate characters. The notochord extends the entire length of the body. They show a dorsal, tubular neural tube without a definite brain.
1. Body is fish -like and is useful for burrowing and swimming.
2. It has a head.
3. It shows a tail.
4. Appendages are absent.
5. Dorsal, caudal and ventral fins are present.
6. Body- wall shows one- cell thick, non-ciliated epidermis, dermis, connective tissue, striated muscle and parietal peritoneum.
7. It has no exoskeleton.
8. Notochord extends from the anterior end to posterior end.
9. Enterocoelic coelom is present. However, reduced in the pharyngeal region by atrium.
10. Alimentary canal is long. It includes a large pharynx with many gill-slits ciliary mode of feeding is developed.
1 1. Gillss will perform respiration.
12. Circulatory system is closed.
13. Heart and respiratory pigments are absent.
14. Hepatic portal system is present.
15. Excretory system shows paired protonephridia with solenocytes.
16. Brain is not present
17. Two pairs of cerebral and several pairs of spinal nerves are present.
18. Sexes are separate. Gonads are metamerically arranged and with out gonoducts.
19. Asexual reproduction will not take place.
20. Fertilization is external.
The sub-phylum Cephalochordata includes a single class- Leptocardii, which has single family, Branchiostomidae.The family contains only two genera Branchiostoma and Asymmetron