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All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail. [8]
The dorsal nerve cord is an anatomical feature found in chordate animals, mainly in the subphyla Vertebrata and Cephalochordata, as well as in some hemichordates.It is one of the five embryonic features unique to all chordates, the other four being a notochord, a post-anal tail, an endostyle, and pharyngeal slits.
The five synapomorphies are pharyngeal slits, a dorsal nerve cord, notochord, endostyle, and the post-anal-tail which is depicted and labeled well on the chordates page. This image is helpful to visualize the regions where the five synapomorphies existed in chordates and what they looked like.
The trunk is the longest part of the animal. It contains the pharynx, which is perforated with gill slits (or pharyngeal slits), the oesophagus, a long intestine, and a terminal anus. It also contains the gonads. A post-anal tail is present in juvenile members of the acorn worm family Harrimaniidae. [6]
Despite their simple appearance and very different adult form, their close relationship to the vertebrates is certain. Both groups are chordates, as evidenced by the fact that during their mobile larval stage, tunicates possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, post-anal tail, and an endostyle. They resemble a tadpole.
Since Ambulacraria is the sister-group of the chordates within the deuterostomes, this suggests that the last common ancestor of both groups lacked a post-anal tail. [16] However, ascidian larvae have been noted to have endoderm extending to the terminal end, which could suggest that tunicates also lacked post-anal tails ancestrally. [11]
Some also have a postanal tail which may be homologous to the post-anal tail of vertebrates. An interesting trait is that its three-section body plan is no longer present in the vertebrates, except for the anatomy of the frontal neural tube, later developed into a brain which is divided into three main parts.
The lancelet is a small worm-like animal with a head end, complete with a mouth and simple feeding structures; a tail end with an anus and post-anal tail fin; a simple digestive tract; a pharynx with gill slits; a dorsal stiffened rod (notochord); V-shaped muscles; and a dorsal nerve chord—all of which are synapomorphies with chordates. B. & C.