Urochordates

Urochordates have a notochord that extends from just behind the head to the tail (rather than from head to tail; Urochordata means "tail-cord"). There are three classes within this subphylum: Ascidiacea, Thaliacea, and Larvacea. Examine the demonstration material as you work through this exercise. Make sketches of your observations in the results section and label as many structures as possible.

• Class Ascidiacea. Adult ascidians (commonly called "sea squirts") are sessile inhabitants of the intertidal zone. Depending on the species, they may be either solitary or colonial and most are monoecious. All are filter feeders. Water with suspended plankton is drawn through an incurrent siphon into the pharynx (Fig 6.2). Organic particulates are strained from the water by the pharyngeal gill slits and trapped in mucus at the base of the pharynx. Ciliary movements carry the food-mucus mixture to the stomach where digestion begins. The filtered water, on the other hand, passes from the pharynx to the atrium and out the excurrent siphon (the intestine also empties near the excurrent siphon). A body wall (or mantle) lines the body cavity and aids in respiration. The outer surface of the animal is covered by a tough tunic (or test; the presence of the tunic furnishes another common name for sea squirts: the "tunicates"). While the tunic of most species is opaque, that of Ecteinascidia is transparent (allowing easy observation of their internal structure).

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Figure 1. Internal organs of an adult ascidian (sea squirt). Large arrows show water flow through the animal; small arrows, the path food trapped by the pharyngeal gill slits takes through the animal.

• Class Thaliacea. Thaliaceans are odd, barrel-shaped things commonly known as "salps". They are surrounded by circular muscle bands and both ends of their cylindrical body are open. Contractions of the circular muscles make the body pulsate; drawing water in the incurrent siphon and forcing it out the excurrent. This endows them both with a form of jet propulsion and a mechanism for filter-feeding (suspended food is removed from the water by the pharyngeal gill slits).
• Class Larvacea. Larvacians superficially resemble ascidian larvae and are the most specialized of the urochordates. They build a peculiar "mobile home" within which they travel throughout their lives (sort of a pelagic Winnebago). The thin walls of their mobile homes afford them little protection from predators (or the ear-splitting heavy metal music of their neighbors), but serve instead to collect and funnel water through their bodies. Like adult ascidians, pharyngeal gill slits are used for feeding. The similarity between ascidian larvae and adult larvacians suggests they may be neotenous urochordates (e.g. the larval forms have attained sexual maturity without loss of larval structures, such as the tail).

Ascidian Larvae	Young Ascidian Larvae	Older Ascidian Larvae	Adult Ascidian

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