A typical sea squirt,or tunicate,is Ciona (Figure 1.2(a)), which lives attached to rocks in seas around the world. It is a 100-150 mm tall bag-shaped organism with a translucent outer skin (the tunic) and two openings, or siphons, at the top. The body is firmly fixed to a hard substrate.
The internal structure is fairly complex (Figure 1.2(b)).A large pharynx fills most of the internal space, and its walls are perforated by hundreds of gill slits, each of which bears a fringe of cilia, fine hair-like vibratile structures. Seawater is pumped through the inhalant siphon into the pharynx by beating movements of the cilia, and the water is then passed through a surrounding cavity, the atrium,and ejected through the exhalant siphon. The pharynx serves mainly to capture food particles from the stream of seawater that flows through it. The seawater is drawn into a filter bag of mucus, which is produced inside the pharynx by a gland known as the endostyle. During feeding, this gland continuously se-
cretes mucus into the oesophagus, together with the food particles that it has filtered from the seawater, and the food is passed to the stomach for digestion.
Why is Ciona identified as a chordate? The pharynx and other structures are in fact very like those of the cephalochordates and lamprey larvae, but further evidence is to be found in the larval stage, when the sea squirt is a tiny free-swimming tadpole-shaped animal with a head and a tail. The larval sea squirt (Figure 1.2(c)) has a notochord that runs along the tail, and this identifies it as a chordate. There are muscles on either side of the notochord that contract alternately, causing the tail to beat from side to side, and this drives the animal forward in the water. The larva has a dorsal nerve cord, running along the tail just above the notochord, and this expands at the front into a very simple brain which includes a light sensor (an 'eye') and a tilt detector.
The larva then settles on a suitable surface. It upends on to the tip of its 'snout' and attaches itself by means of adhesive suckers (Figure 1.2(d)). The noto-chord and tail portion wither away, and the pharynx and gut expand to fill up the body cavity. This extraordinary metamorphosis occurs rapidly to allow the adult to start feeding in its new way as soon as possible.
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