Amphioxus was first described and discovered by Pallas (1778), who regarded it a kind of slug (mollusc) and named it Limax lanceolatus. An Italian naturalist, Costa (1834) was the first to identify it correctly as a low vertebrate and called it Branchiostoma lanceolatum. A couple of years later Yarrel rediscovered if and gave the most popular name Amphioxus, a name that has come to be nearly universal, although the law of priority claims the use of the name Branchiostoma. Yarrel included it with fishes in the order Marsipobranchii, but any animal without gills, heart, brain, endoskeleton and paired eyes and other sense organs cannot be grouped under Pisces. Besides these characters, Amphioxus possesses colourless blood and its pharynx is surrounded by the atrium. Therefore a new group Cephalochordata was established for Amphioxus and the same has been raised to the status of a subphylum now.
From the study of morphology and anatomy of Amphioxus it is evident that it
possesses practically the full complement of chordate characters and differs from the craniates in the relative simplicity of its bodily organization. The opinions however are divided on its position. Some regard it as primitive whereas, others consider it to be modified or degenerate.
The primitive features which Amphioxus seems to share with the prechordate ancestor include: (i) the naked notochord from end to end; (ii) the complete myotomic segmentation from end to end; (iii) the absence of jaws for biting; (iv) the absence of paired fins; (v) simple one-layered epidermis; (vi) the simple straight tubular intestine with a very simple liver diverticulum and the ciliary method of feeding (without its extreme specialization); (vii) the simple circulatory system without a specialized heart; (viii) segmental nephridia, which are not coelomoduct; (ix) segmental gonads with no ducts; (x) the separate dorsal and ventral roots of spinal nerves; (xi) small almost yolkless egg, hollow spherical blastula, embolic gastrulation and formation of anterior coelomic pouches.
Amphioxus also possesses certain characters which are supposed to be specialized features. The peculiar pharynx, for instance, is the most important of such characters.
It has very large number of gill-slits, which have increased far beyond the number of metameres that carry them and have probably multiplied by secondary polyisomerism.
Most ciliary feeders rely on their peculiar pharynx for the maintenance of a
current of water from which the food is sifted. The collection of sufficient amount of food particles is possible when large amount of water passes. Increase in the number of gill-slits makes this possible. The food sifting is made possible by the presence of the elaborate velum and oral hood equipment. Besides these, the atrium and atriopore and the distortions of the coelom associated with the atrial system, constitute other such specialized features.
The degenerate characters of the brain include: (i) the reduced brain and sense organs probably in response to the assumption of sedentary life; (ii) the extension of the notochord up to the anteriormost tip of the body.
Amphioxus has been regarded as a generalized chordate. This has been very clearly expressed by the statement, “That if Amphioxus had not been discovered it would have to have been invented.” Its structure throws much light on the form of ancestral chordate, but unfortunately gives no hint of the way in which it might have evolved from any known type of non-chordate animal.
Recent studies, however, link them with the tunicates. It has already been pointed out that there exists a similarity in the mode of development in the nervous system in ascidian tadpole and the Amphioxus larva. The notochord appears similarly in both and presents similar structure. The pharynx with its complicated equipment including the endostyle, epipharyngeal groove (dorsal lamina), etc., is similar in both.
Atrium and atriopore in the larval forms are similar. The natural conclusion is that the two groups (the Tunicata and Cephalochordata) show impressive correspondence in their structure and embryonic development and, evidently enough, possess common ancestry.