CEPHALOCHOKDA 



137 



protect the delicate structures of the gill region, but allow 

 of muscular contractions taking place in that region which 

 occupies so large a share of the body. 



The pharyngeal apparatus serves to filter off the food from 

 the water, and as a respiratory organ. The cilia of the oral 

 cavity bring about a constant current of water. There are 

 no jaws, and the food in suspension is drawn into the pharynx. 

 The endostylar glands secrete mucin which entangles the food, 

 the cilia of the endostyle form the mucin into a rope, and this, 

 with the aid of the 

 peripharyngeal bands in 

 front and the cilia of the 

 gills over the rest of the 

 pharynx, is gradually 

 carried upwards to the 

 epipharyngeal groove. 

 The food is carried along 

 the epipharyngeal groove 

 to the posterior end of 

 the pharynx, and so to 

 the stomach. 



The stomach is de- 

 fined from the intestine 



Only in being slightly FlG> 63. Amphioxus. Transverse section 

 Wider and by its blind in region of pharynx. 



outgrowth on the right 



side. This' is the liver, or hepatic caecum, the blind end of 

 which is directed forwards on the right side of the pharynx. 

 It carries with it a covering of ectoderm from the atrial wall. 

 In the young it is short, but in the adult it reaches to the 

 anterior end of the atrium. The epithelial cells of the caecum 

 are columnar and glandular, and they are liable to be thrown 

 off into the cavity and renewed. Digestion and absorption 

 are performed in the caecum. As the cells contain granules, 

 it is believed that the organ also takes a share in excretion. 



The stomach, like the pharynx, is lined by ciliated cells, 

 and these are continued into the intestine. The latter pursues 

 a straight course to the anal opening, which, as has already 

 been observed, occurs on the left side of the ventral fin. 



Dorsal fin 



Nerve cord 

 Notochord 



Dorsal aorta 



Epipharyngeal groove 



Body cavity 



Gill 



Hepatic vein 



Liver 



Myotome 



Atrial ectoderm 



Gonad 

 Ventral aorta 



'Metapleure 



