Subphylum: Cephalochordata

• Body fish-like adapted for burrowing and swimming. It lacks head, but possesses a tail.
• Paired appendages absent, median fins (dorsal, ventral and caudal) are present.
• Notochord extends the entire length of the body and passes ahead of the nerve cord in front. Notochord persistent throughout life.
• A true enterocoelous coelom is present. It is however reduced in the pharyngeal region of atrium.
• Digestive tract complete; circulatory system closed; heart and respiratory pigments are wanting.
• Pharyngeal gill-slits more numerous and better developed.
• Brain is indistinct; no paired sense organs - eyes, ears, nares.
• Excretory system includes paired protonephridia with solenocytes.
• Myotomes are the segments of muscles.
• Sexes, separate; fertilization external; holoblastic segmentation.
• Larva undergoes progressive metamorphosis. e.g. Branchiostoma (Amphioxus).

Fig. Branchiostoma : Entire animal in right side view