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muscles ; these, and the row of sac-like gonads, partly 

 visible below segments 10 to 35, show the segmentation of 

 the body. Along the back there is a slight median fold 

 of skin, the dorsal fin ; this is extended round the tail as 

 a wider caudal fin, and is continued ventrally, a short 

 distance, as the narrow anal fin. The dorsal and anal fins 

 are stiffened by numerous small fin rays. 



Under the anterior end of the body is the oral hood, 

 which encloses the buccal cavity ; its external opening, 

 fringed with ciliated processes (buccal cirri), is the buccal 

 opening, often called the ventral mouth. The true mouth 

 is the aperture of the velum, which is "a muscular dia- 

 phragm between the buccal cavity and the pharynx " 

 fringed with twelve tentacles. The flattened ventral 

 surface of the anterior two-thirds of the body is bordered 

 by two lateral metapleural folds, which extend from the 

 oral hood and meet together behind the atrial pore in 

 front of the anal fin. The ventral anus opens midway 

 between the atriopore and the tip of the tail, slightly to 

 the left of the middle line. 



Amphioxus lanceolatus is found in the shallow coastal 

 waters around Britain (English Channel), North Sea, 

 Mediterranean, and other seas. It 'burrows in the sand, 

 habitually lying buried with only the mouth protruding. 

 It also swims actively (by wriggling). 



How does Amphioxus feed ? Explain the Structure of the 

 Alimentary Canal of Amphioxus. . Which part of 

 the Alimentary Canal is respiratory, and how is 

 Respiration effected ? 



The buccal cavity (stomodaeum) has a ciliated lining, 

 certain parts of which have specially long cilia and form 

 the " wheel-organ " ; by that means the indrawn water 

 is driven into the pharynx. The small organisms and 

 particles in the water become entangled in the mucus 

 which is secreted in the endostyle (the mid-ventral groove 

 along inner wall of pharynx) and moved on up the sides 

 of the gill-slits and the anterior peripharyngeal bands 

 (behind the velum) into the mid-dorsal epipharyngeal 

 groove which opens into the stomach. In this way Amphi- 

 oxus feeds. 



The large pharynx forms about half the length of the 

 alimentary canal ; the other half is the intestine, a straight 

 tube ending in the* anus. A pouch-like outgrowth, from 

 the swollen anterior (stomach) portion of the intestine, is 

 the " liver," which is directed forwards on the right side. 

 Porsally the pharynx is attached to the sheath of the 



