The urochordates are marine deuterostomes related to the cephalochordates and vertebrates. They are suspension feeders and most use on pharyngeal gill slits for feeding. At some time in the life cycle all possess a notochord and dorsal hollow nerve cord which, along with the gill slits, are the defining characteristics of the phylum chordata. There are four classes, of which the Ascidiacea is usually taken as representative for the purposes of introductory laboratory exercises. The ascidians, or sea squirts, are sessile filter feeders that, as adults, bear little resemblance to their chordate relatives.
Ascidians have a living, external, cellular exoskeleton, or tunic, underlain by the epidermis.
The tunic resembles connective tissue, except that it is outside the epidermis, and consists of cells, a secreted extracellular matrix, and ground substance. Much of it is a cellulose-like polysaccharide. In many ascidians blood vessels cross the epidermis to enter the tunic. The gut is U-shaped and both openings are anterior, with the anus dorsal to the mouth. The gut is dominated by an enormous pharynx whose wall is perforated by numerous tiny gill slits.