6z ORIGIN OF CHORDATES in. 4 



upwards and is then passed back to the oesophagus by the cilia of a 

 dorsal lamina or of a series of hook-like 'languets'. Autoradiographs 

 made from tunicates that have been provided with isotopes of iodine 

 show that iodination occurs in certain cells lying above the glandular 

 tracts of the endostyle. Iodine is also abundant in the tunic, as it is in 

 the exoskeletal structures of molluscs and insects. When it became 

 of metabolic value its production may have become concentrated in 

 the pharynx (see p. 118). 



The extensive ciliated surface of the pharyngeal wall ensures the 

 passage of large volumes of water inwards at the mouth and out at the 

 atriopore. Rapid change of the water is also produced by periodic 

 muscular contractions (p. 65). The pressure of the exhalant current is 

 sufficient to drive the water that has been used well away from the 

 animal. 



The oesophagus leads to a large 'stomach' with a folded wall con- 

 taining gland-cells, which produce digestive enzymes. These include 

 much amylase, invertase, small amounts of lipase, and a protease of 

 the tryptic type. The organ is therefore not to be compared with the 

 stomach of vertebrates. A branching 'pyloric gland' opens into the 

 lower end of the stomach. From the stomach a rather short intestine 

 leads upwards to open inside the atriopore; this is apparently the 

 absorptive region of the gut. 



The heart lies below the pharynx and is a sac, surrounded by a 

 pericardium (see p. 63) and communicating with a system of blood 

 spaces derived from the blastocoele. The larger of these spaces have 

 an endothelial lining; the biggest is a hypobranchial vessel below the 

 endostyle, from which branches pass to the pharynx. From the oppo- 

 site end of the heart springs a large visceral vessel and others pass to 

 the dorsal side of the pharynx, tunic, body wall, &c. The heart is 

 peculiar in that the beat can proceed in either direction. After passing 

 blood into the hypobranchial vessel and gills for a few beats, its direc- 

 tion reverses, passing the blood to the viscera. This reversal is pro- 

 duced by the presence of two pacemaker centres, each capable of 

 initiating rhythmical contractions, one at either end of the heart. 

 Stimulation of these by warming and cooling allows control of the 

 reversal of the beat. There are no capillaries and the blood system is 

 a haemocoele. The blood-plasma is colourless but contains corpuscles, 

 some of which are phagocytes, while others contain orange, green, or 

 blue pigment (in different species). The green and other pigments are 

 remarkable in that they contain vanadium. In some ascidians (Molgula) 

 some individuals contain vanadium, others niobium (Carlisle, 1958). 



