Amphioxus is included in:
Phylum – Chordata
Group – Protochordata
Sub-Phylum – Cephalochordata
The body is elongated, fish like, semi-transparent, laterally compressed and pointed at both ends. It measures about 5-7 compound in length, the colour of the body is dark red and reddish brown.
The body of the amphioxus is divided into two parts: The trunk and tail. The head is absent. The paired fins are absent, but unpaired fins such as a dorsal fin, running along the whole length, a caudal fin, around the tailed a ventral fin, are present.
The dorsal fin is supported by a series of fin rays. But these are not similar to fin rays of fishes. The trunk bears three openings: the mouth atripore and anus. The anterior end of trunk is pointed called rostrum. The mouth lies below the rostrum. The mouth leads into oral hood formed by dorsal and lateral projections of the body. The edge of the oral hood is provided with 10 to 11 pairs of oral cirri or tentacles. The cirri helps in filter feeding and draw water current towards pharynx.
The oral hood encloses a cup shaped buccal cavity called vestibule. The atripore is a small mid-ventral aperture situated in front of the ventral fin. The anus lies at the base of caudal fin on the ventral side. From the posterior part of the oral hood two lateral membranous metapleural folds run up to the atriopore.