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(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classification descriptions. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM Copyright 2002 ETI / Freeman & Co Publishers)

Tunicates are considered acraniate chordates because tunicates and chordates have the following features in common: a notochord; a dorsal, hollow nerve cord; and pharyngeal gill slits at some time in their lives. The notochord is a stiff cylinder of cells, each cell containing a fluid-filled vacuole. The notochord of urochordates extends the length of the body but does not persist throughout life except in Class Larvacea. The embryonic notochord of vertebrates is replaced in adult vertebrates by cartilage or bony vertebrae as support. The hollow, dorsal nerve cord is resorbed by tunicates at metamorphosis. Adult tunicates have a small cerebral ganglion but no brain. In vertebrates, the nerve cord becomes the spinal cord and brain. Prominent gill slits are present in both larval and adult tunicates; whereas, in many vertebrates (reptiles, birds, mammals, and most amphibians), gill slits evident in the embryo are transformed and closed in the adult. Fish and a few amphibians retain their gill slits into adulthood. Acraniate chordates—tunicates and cephalochordates (Phylum Cephalochordata)—are considered by some to be subphyla within Phylum Chordata.
All urochordates are marine and comprise about 90 percent of the invertebrate (acraniate) chordates. Adult urochordates secrete an external tunic that usually (except in larvaceans) contains tunicin, a polysaccharide related to cellulose. Within the tunic is the body, which contains a pharynx that filters phytoplankton from incoming seawater and exchanges oxygen and carbon dioxide with the seawater. Urochordates use three different mechanisms for generating a seawater current through their bodies, according to their class: beating of cilia on the pharynx (ascidians), beating of the tail to drive water through the “house” (larvaceans), and contraction of circular muscle bands and beating of cilia (thaliaceans). The urochordate continually pumps water into the body, through its pharynx, and out of the body. Fertilization usually takes place in the sea for solitary ascidians and larvaceans and is internal for colonial ascidians and thaliacians. The 1400 species of tunicates grouped in three classes—Ascidiaceae, Larvacea, and Thaliacea—are significant members of oceanic food webs. Some sea stars, for example, feed on tunicates.
Members of class Ascidiaceae—commonly called sea squirts, tunicates, or ascidians—have free-swimming, nonfeeding larvae and benthic (seafloor dwelling), sessile adults from 1 to 120 mm long. A few species of ascidians live in soft sediments in beds of sea grass, attached by stalks or filaments to shells or underwater cliffs, and some (Dicarpa) are members of the community of the abyss. The sea squirt larva (tadpole) has adhesive suckers (papilli) on its head and a muscular tail, like the frog tadpole that it superfically resembles. The larval nervous system includes a dorsal pigment-containing light receptor called an ocellus and a dorsal, hollow nerve cord (neural tube) that runs the length of its tail. The ascidian larva has a nonfunctional digestive tube. After a day of swimming, ascidian larvae adhere with their anterior ends to the bottom—ocean floor, dock pilings, or ship hulls—and undergo extensive metamorphosis into the sessile adult. Tail, nerve cord, and notochord are resorbed as the body rotates about 180 degrees; the oral siphon migrates so as to open opposite the attachment to the substrate; other internal organs also rotate to their adult locations. In the adult ascidian, seawater enters the oral siphon, moves through slits in the pharynx, and leaves through the excurrent siphon; this feeding current is generated by cilia on the pharynx.
Epidermal cells secrete the tunic, which is transparent or purple, red, pink, orange, yellow, or green; the ascidian tunic may be rough, smooth, or hairy or it may bear stiff spicules. The spicule-stiffened tunic and pharyngeal basket (feeding and respiratory structure) are mechanically analogous to pliable synthetic material reinforced with stiff inclusions. The tunic spicules of Herdmania form in tunic blood vessels (separated from blood by an envelope that may be a cell) and migrate through the blood vessel and through the tunic to project from the surface of the tunic. Blood cells and, in some species, blood vessels are located in the vase-shaped tunic itself. The blood carries nutrients and oxygen. The circulatory fluid contains no oxygen-binding pigment but does have morula cells (clusters of cells that resemble a mulberry or raspberry) that accumulate vanadium (perhaps a defense mechanism) and deposit it in the tunic. The tubular heart of the ascidian adult is located in the atrium, a cavity between tunic and pharynx. The urochordate heart has a pacemaker at each end. Every few minutes, the heart reverses the direction of its pumping. Cells in the tunic conduct impulses and contract, but the tunic lacks nerves and muscles. Just inside the tunic is a body wall with muscle bands; muscle bands are more extensively developed in thaliaceans. When an ascidian tunicate is disturbed, it contracts the muscle bands in its body wall and closes both siphons. This contraction around the trapped internal water causes the tunic to become turgid, which forces the tunic spicules to project outward, possibly deterring potential predators.
The ciliary feeding and respiratory structure—pharyngeal basket—takes up the greater part of the body of the ascidian tunicate adult; water is pulled in through the oral siphon by cilia (in ascidians). Dissolved oxygen diffuses into the tunicate as the feeding current passes through the pharyngeal gill slits. As the water passes out of the pharynx through these pharyngeal perforations, food particles collect on a mucus sheet secreted by the endostyle—a ciliated groove just below the incurrent (oral) siphon. Then, tiny tentacles (in some species) roll the food-coated sheet into a rope that cilia transport to the esophagus opening. Food is digested extracellularly and absorbed as it moves through esophagus, stomach, and intestine. The filtered water moves through pharyngeal slits into the atrium and out the excurrent siphon. Contraction of body-wall muscles squirts out uncollected food bits and water, giving rise to the common name sea squirt. Waste particles leaving through the anus and gametes leaving through the genital pores are expelled with the outgoing current from the excurrent siphon. Colonial ascidians sometimes share a common excurrent siphon. In most urochordates, most dissolved waste—ammonia is removed by diffusion; the ascidian Molgula accumulates uric acid in renal sacs. The tunicate Lissoclinum, a coral reef tunicate, hosts a photosynthetic bacterium Prochloron (see chloroxybacteria, Phylum B-5) in the wall of its combined excretory and reproductive canal. (In many tunicates, anus and gonopores are separate.)
Ascidian tunicates are simultaneous hermaphrodites with male and female genital pores opening near the anus. Most species release gametes, and fertilization usually takes place in the sea (Ciona) but is internal in colonial ascidians. Some ascidians (Botryllus) form sessile colonies in a common gel tunic by budding. In other ascidians, buds separate from the parent.
All members of class Larvacea (Appendicularia) are pelagic and solitary, floating free in the open ocean worldwide. Some species of Oikopleura bioluminesce. Bathycordius can reach 8 cm or more; most larvaceans are smaller than 10 mm in length. The larvacean tunic is not of tunicin. The larvacean adult looks like an ascidian larva; the larval form is retained in adulthood, but the larvacean is a mature, tailed adult with a notochord (an example of retention of larval structure in the adult) and gonads. Like ascidian tunicates, adult larvaceans bear both ovary and testis, heart, and digestive tract; but, unlike ascidians, they form neither buds nor colonies. The adult secretes a soft “house” (case) around itself. Through this house, the larvacean drives ocean water by lashing its tail. Water enters the porous house through mucus-covered external filters that retain food particles; the food is passed to a second, pharyngeal filter and to the mouth and digestive tract. Eventually, water exits through the excurrent siphon. Feces exit through the anus and collect in the soft case. When its external filters clog, the larvacean first secretes a new house, then exits from the old house through an escape hatch, and inflates a new house around itself. The larvacean changes houses several times a day. Although the most abundant of the free-living tunicates, the larvaceans are seldom seen except by an observer in a remote-operating vehicle or by divers, because the fragile larvaceans are destroyed by tow nets.
Members of class Thaliacea, such as Salpa, commonly known as salps or chain tunicates, have complex life cycles that alternate between solitary sexual forms and colonial asexual forms. For example, an adult Doliolum may bud hundreds of individuals in a chain; these individuals eventually separate as sexually reproducing adults that produce larvae. Large concentrations of Pyrosoma and other thaliaceans are a frequent source of ocean bioluminescence. Thaliaceans are free-living translucent urochordates most common in warm ocean surface water. Pyrosoma colonies may extend several meters through the sea; individuals range from a few millimeters to 24 cm in length. Their barrel-shaped bodies are banded by muscle, much as hoops band a wooden cask; incurrent and excurrent siphons open at opposite ends of the body, and the muscle bands generate locomotion. The doliolid thaliacean is jet propelled; by shutting one siphon and contracting its circular muscles, it shoots water out of its rear siphon and zips along at a rate of as much as 50 body lengths per second. The anatomy with respect to muscles of doliolid thaliaceans such as Doliolum is like that of salps such as Salpa but with complete muscle bands instead of incomplete bands; neither ascidians nor larvaceans have such muscles. Like ascidians and larvaceans, members of Class Thaliacea are ciliary filter feeders; they secrete mucus with an endostyle and capture food on a slitted, mucus-coated pharyngeal bag or a pharynx reduced to a ciliated bar.
Ascidian spicules are fossilized but the urochordate soft bodies have left no recognizable fossils. Urochordate larvae have left evolutionary footprints—the notochord, pharyngeal gill slits, and hollow, fluid-filled, dorsal nerve cord common to all chordates.

Subphylum Urochordata