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IN CONTEXT: UROCHORDATA The Appendicularia are a class of pelagic tunicates (Subphylum Urochordata) that, while little-known to most people, are nonetheless of considerable importance to the marine systems in which they live. Like their more familiar cousins, the sea squirts (tunicate Class Ascidiacea), appendicularians begin life as tadpole-like larvae resident within the plankton. After a short stint as meroplankton, larval sea squirts undergo metamorphosis to become the sessile, tough-skinned, benthic animals recognizable as sea squirts. In contrast, appendicularians are holoplanktonic, spending their entire lives in the water column. 
Larval sea squirts are interesting in that they are not yet capable of feeding.
Their thick outer body covering (the tunic from whence comes the subphylum common name
"tunicate") completely covers the incurrent mouth and excurrent atriopore - the opposing business
ends of the postlarval feeding and digestive apparatus. Adult appendicularian "tadpoles,"
on the other hand, are highly efficient filter-feeding animals, as described in the
Focus on Appendicularia mission report.
Larval tunicates are also intriguing because they exhibit all four of the developmental hallmarks that distinguish them as belonging to Phylum Chordata - the same phylum as humans. 
For at least part of their life, all chordates (including humans) possess these four structural features: a notochord; a hollow, dorsal nerve cord; pharyngeal gill slits; and a post-anal tail. We lose our gills, notochord, and tail during embryogenesis; sea squirts keep their pharyngeal gills, but lose their tails, notochords, and nerve cords at metamorphosis. The similarities between larval urochordates and "advanced" members of the chordate phylum (i.e., Subphylum Vertebrata) have fueled more than a century of investigation and debate as to the origins of the vertebrates. With its propulsive muscular tail, notochord, and dorsal nerve cord, the urochordate larval form presages beautifully the vertebrate line. But from before the turn of the 21st Century, evolutionary biologists grappled with various theories seeking to bridge the gap between tunicates and vertebrates. Originally, sessile adult sea squirts were viewed as degenerate forms of a hypothetical chordate ancestor which was envisioned as a sexually mature version of the urochordate tadpole larva. 
But paleobiologist Walter Garstang turned the tables on conventional wisdom. In a 1928
publication, Garstang suggested that the uneqivocally chordate tadpole larvae of the
tunicates was an evolutionary answer to the needs of sessile adult forms to
effectively disperse offspring to new habitats and locations. He went on to
propose that at some point the tadpole larvae of one urochordate line became
paedogenetic - exhibiting sexual maturity and reproducing while in the larval stage. Ultimately, this line would evolve into the free-swimming vertebrates.
Although the exact sequence of evolutionary events remains speculative, the modern appendicularians are a key piece of evidence in support of Garstang's original theory. Although they are extremely specialized, particularly in regard to their feeding strategy, present-day appendicularians are still basically paedogenetic tunicates. Perhaps this fact has never been stated so eloquently as it was by Garstang himself in a verse from his 1958 Larval Forms, and Other Zoological Verses entitled "Oikopleura, Jelly-builder." The unusual combination of larval and adult features exhibited by appendicularians and a few other animal taxa is termed neoteny, in reference to early life history traits that are retained in the adult form. The neotenic condition of appendicularians is integral, not incidental, to the success of the group. The unique appendicularian feeding apparatus is powered by the beating of its muscular tail. 
There is a third class of tunicates. These are the holoplanktonic salps and doliolids, members of Class Thaliacea. Members possess transparent, spindle-shaped or cylindrical bodies, with incurrent and excurrent siphons located at opposing ends of the body. Although the thaliaceans are largely passively transported by currents, the body wall contains bands of circular muscle which can contract to provid a limited degree of jet propulsion.  | |
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