Amphioxus burrows in clean shifting and shallow shore waters leaving only its anterior end protruded. For most of the times, it remains buried in the sand but in darkness it swims very rapidly by lashing movements of the tail. It is a ciliary feeder.

GENERAL CHARACTERSTICS OF AMPHIOXUS

Commonly called as lancelet.
It was first discovered by Pallas (1778).
Adult is less than 5 cm long and has superficially fish-like structural make up.
Body is elongated, flattened, non-pigmented and pointed at both ends as the name implies. Anterior end projects forwards as the rostrum.
Dorsal, ventral and caudal fins are low and continuous. There are 2 lateral fins or metapleural folds. Dorsal fin has fin rays.
Ventral mouth is guarded by oral hood containing oral cirri.
Atriopore is median and ventral. Anus on left side.
Myotomes are arranged on both sides of die body as metamerical blocks of striated muscle fibres separated by V-shaped partitions called as myosepta or myocommata.
Notochord is an axial skeletal rod extending from anterior to posterior end. Nerve cord lies just above the notochord.
Gonads 26 pairs, metamerically arranged on pharynx. The two sexes are separate but without sexual dimorphism.

SPECIAL FEATURES

Although eyes, nose, ears, jaws and appendages are completely absent, yet Amphioxus is of special zoological interest, because it shows three distinctive characters of the phylum Chordata in simple form i.e., presence of notochord, nerve cord and gill-slits. It is considered to resemble some ancient ancestor of the phylum Chordata. Further Amphioxus shows combination of primitive, specialized and degenerate features.
Primitive features include notochord extending into snout, segmented myotomes, straight intestine, absence of jaws and paired fins, ciliary feeding and no specialized heart.
The specialized features comprise of elaborate velum and oral hood and several gill-slits. The reduced brain and sense organs are degenerate features. demonstrated true position of the group among chordates. The tunicates are best understood by studying first free-living larva of an ascidian, than the adult. The larva shows chordate characters but same are absent in adult and others are modified to sessile mode of life. Compound ascidians reproduce asexually by gemmation and also produce eggs and sperms for sexual reproduction.

IDENTIFICATION

The animal contains oral hood, spindle-shaped body and myotomes and above features, hence it is Amphioxus.

REFERENCES

https://en.wikipedia.org/wiki/Lancelet
https://www.alamy.com/stock-photo/branchiostoma.html
Book of Practical Zoology VERTEBRATE by S.S.LAL – Rastogi Publications.

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