(1) One such taxon is the Urochordata or tunicates, a highly diverse group of organisms that live in the widest variety of marine environments spread throughout the globe.(2) For this reason, the three subphyla, Urochordata , Cephalochordata, and Vertebrata, will be treated separately.(3) The Urochordata , sometimes known as the Tunicata, are commonly known as ‘sea squirts.’(4) Some zoologists would put the Urochordata closer to the Echinodermata, instead of within the Chordata proper.(5) For instance, adult urochordates are typically sessile, while many vertebrates are quite active.(6) In particular, there is a significant phylogenetic gap in information for the urochordates , a subphylum near the invertebrate-vertebrate transition.(7) The urochordates or tunicates are sessile (attached to the bottom) marine filter-feeders, and rather sponge-like as adults.(8) Chordates are divided into urochordates , cephalochordates, and vertebrates.(9) Although this classification is widely accepted, some authors recognize three chordate phyla: Urochordata for the urochordates, Cephalochordata for the lancelets, and Vertebrata for the vertebrates.(10) Their report that urochordates are comprised of 4 discrete lineages holds interesting implications for understanding the evolution of tadpole morphology and chordate life history.
