Q.Give the chordate-identity features of tunicates (Urochordata).

[!TLDR] Tunicates are chordates because their free-swimming larva shows all four chordate hallmarks, even though the adult does not.

[!ANSWER] The chordate identity of tunicates rests on their larval stage: the free-swimming tunicate larva shows a notochord (confined to the tail), a dorsal tubular nerve cord, pharyngeal gill slits, and a post-anal tail, even though the sessile adult loses most of these and retains only an enlarged pharyngeal gill-slit basket and the endostyle.

[!ANSWER] Tunicates (Urochordata) qualify as chordates because of features expressed in their larva, not their adult.

The adult tunicate is a sessile, saclike, filter-feeding animal that shows no obvious notochord, no post-anal tail, and only a small ganglion in place of a proper dorsal nerve cord — on its own, the adult gives little indication of belonging to Chordata. The free-swimming tunicate larva, however, clearly displays all four chordate hallmark features: a notochord, present specifically in the tail region of the larva; a dorsal, hollow nerve cord; paired pharyngeal gill slits; and a post-anal tail, giving the larva a superficial resemblance to a small tadpole.

When this larva undergoes metamorphosis and settles to become the sessile adult, the notochord, tail, and most of the nerve cord are lost or drastically reduced, while the pharyngeal gill slits are retained and become greatly enlarged into a basket-like structure used for filter feeding, alongside the endostyle in the pharynx floor, which continues the food-trapping role in the adult.

So the tunicate's chordate identity is established specifically by demonstrating all four hallmark chordate features in its larval stage, even though the adult body form, on its own, would not obviously suggest a chordate affinity.

[!ANSWER] Tunicates are classified as chordates because their free-swimming larva (not the sessile adult) clearly shows all four chordate hallmarks — notochord, dorsal nerve cord, pharyngeal gill slits, post-anal tail — most of which are lost at metamorphosis into the adult.

• Looking for chordate features in the ADULT tunicate — the identifying hallmark features belong to the larva.
• Forgetting that the pharyngeal gill slits, unlike the notochord and tail, are actually retained (and enlarged) into the adult.