32 CHORDATE ORGANIZATION n. 7- 



dorsal epipharyngeal groove, in which it is conducted backwards. 

 The cilia of the endostyle also move mucus along the peripharyngeal 

 ciliated tracts, behind the velum, to join the epipharyngeal groove. 

 Radioactive iodine is concentrated by one of the columns of the endo- 

 style and secreted with the mucus. Barrington (1958) suggests that 

 these may be regarded as the precursors of the thyroid cells, serving 

 to produce iodinated mucoproteins, which are then absorbed farther 

 down the gut (see p. 119). 



The pharynx narrows at its hind end to open dorsally into a region 

 best known as the mid-gut, the name stomach being inappropriate. 



£{■ nig. ant. fg.post (cft 



/ Ag 



T ^^^' |l ^ | i | Mii)iifmiTn^ r . y 



Fig. 10. Currents in the mid-gut of amphioxus, showing the appearance when an 

 animal is placed in a medium containing carmine particles. Arrows show the chief 



ciliary currents. 



div. diverticulum ;f.c. food cord; h.g. hind-gut; i.c.r. ileo-colon ring; m.g.ant. and m.g.post. 



anterior and posterior parts of mid-gut; oes. oesophagus. (After Barrington.) 



A large mid-gut diverticulum reaches forward from this region on the 

 right-hand side of the pharynx. From its position this organ is often 

 called the liver, but Barrington has given reasons for supposing that 

 it is the seat of the production of digestive enzymes. Zymogen cells, 

 similar to those of the mid-gut, are found in its walls. Its strong dorsal 

 and ventral ciliation maintains in it a circulation of food materials and 

 secretion, and its cells are capable of phagocytosis as well as secretory 

 activity. Amphioxus thus combines intracellular with extracellular 

 digestion, doubtless in connexion with its microphagous habit. 

 Particles placed in the diverticulum are swept backwards and join the 

 main food cord that passes through the mid-gut (Fig. 10). 



The hind end of the mid-gut is marked by a specially ciliated region, 

 the ileo-colon ring, whose cilia rotate the cord of mucus and food. The 

 movement is transmitted to the portion of the food cord in the mid- 

 gut and presumably assists in the taking up of the enzymes that 

 emerge from the diverticulum. Extracellular digestion takes place in 

 the mid-gut and the enzymes responsible have been studied by Bar- 

 rington. The pH of the contents varies from 67 to 7-1. An amylase is 

 present in extracts of the diverticulum, mid-gut, and hind-gut, but 

 not in those of the pharynx. Lipase and protease are present in the 



