Q.Distinguish between a sea squirt and a lancelet.

[!TLDR] A sea squirt (tunicate) shows chordate features only as a larva and becomes a sessile, saclike adult; a lancelet keeps all four chordate features as a free-living adult. …

[!ANSWER] Sea squirts and lancelets illustrate two contrasting patterns of chordate character expression across the two invertebrate chordate subphyla.

A sea squirt (a tunicate, subphylum Urochordata) begins life as a free-swimming, tadpole-like larva showing all four chordate hallmarks, but at metamorphosis becomes a sessile, saclike, filter-feeding adult that loses the notochord, tail and most of the nerve cord, retaining only an enlarged pharyngeal gill-slit basket and the endostyle. Its body form as an adult, in isolation, would not obviously suggest a chordate. …

• Assuming both groups show the same pattern of chordate character expression — sea squirts and lancelets differ specifically in whether the adult retains the hallmark features. …