CYCLOSTOMI. 435 



the Myxinoidei. The mouth ( = stomodaeum) is separated from the pharynx 

 by a depending fringed velum in the Myxinoidei. The Ammocoetes has a 

 right and left velar fold ; but in the adult a valve with seven projecting fila- 

 ments overhangs the entrance to the bronchus below the oesophagus (infra) 

 and connects the remnants of the two velar folds which project backwards 

 into it. The alimentary canal is straight. The fore-gut ( = oesophagus and 

 stomach) is narrow, the mid-gut ( = small intestine) begins with a sudden 

 dilatation, into which opens the bile duct; and the hind-gut (= large 

 intestine and proctodaeum) is very short. The mid-gut is separated from 

 both fore- and hind-gut by a valve, and in the Lampreys contains a project- 

 ing fold or typhlosole, which, beginning on the dorsal side of the intestine, 

 makes a slight spiral twist and ends on the ventral side, and is richly 

 supplied with blood-vessels. The epithelium of the mid-gut is ciliated 

 throughout in Ammocoetes , but in the Lamprey only here and there. The 

 liver consists in the Hags of two separate lobes, each with its own duct and 

 a gall-bladder appended to the point where the ducts fuse and form a 

 common duct. It is united to the ventral abdominal wall in Lampreys, and 

 in the Ammocoetes has a tubular structure with ciliated bile-ducts. At the 

 metamorphosis the tubular structure is lost ; fat appears in the cells ; the 

 gall-bladder and bile-duct are absorbed. The mid-gut atrophies more or 

 less at the same time. The pancreas is perhaps represented in the 

 Lampreys by an acinous gland opening into the widened commencement 

 of the mid-gut on the left side. The anus lies in front of the urogenital 

 sinus in the adult l . 



The respiratory system consists of gill-pouches or sacs, seven on each 

 side in the Lampreys ; six, or very rarely seven, in Myxine ; six or seven 

 in Bdellostoma. An eighth, or anterior pouch, is indicated in the embryo 

 Lamprey, but disappears. It corresponds probably with the spiracular 

 cleft of Elasmobratichii. In the Myxinoidei and the Ammocoetes these 

 pouches open internally into the oesophagus ; in the Petromyzon into a 

 tube or bronchus underlying the fore-part of the oesophagus, and ending 

 blindly behind. This bronchus is said to be the oesophagus of the 

 Ammocoetes, the oesophagus of the adult being a new structure which 

 grows forwards from behind at the metamorphosis, and opens into the 

 pharynx above the bronchus. Each branchial pouch has a separate 

 external opening in the Lampreys and Bdellostoma, and the separate 

 openings in the Ammocoetes are united by a deep furrow. In Myxine there 

 are only two ventrally-placed external openings, with which the pouches 

 communicate by separate tubes. A ductus oesophageo-cutaneus, or tube, 

 arising from the oesophagus behind the last pouch on the left side, opens 

 externally with the left common aperture in Myxine, with the last pouch of 

 the left side in Bdellostoma. It is not found in Lampreys. The inner 



1 The anus represents the blastopore in P. Planeri (Shipley, P. R. S. xxxix. 1885). 



F f 2 



