90 VERTEBRATES WITHOUT JAWS iv. 6- 



Little is known of the habits of lampreys in the sea, but in North 

 America there are races of lampreys that are land-locked and feed on 

 the fishes in the lakes, where they have recently become a most serious 

 pest (Fig. 55). 



The oesophagus (fore-gut) leads directly into a straight intestine 

 (mid-gut); there is no true stomach in lampreys (Fig. 56). The surface 

 of the intestine is increased by a typhlosole, running a somewhat 

 spiral course. There is a liver, gall-bladder, and bile-duct of typical 

 vertebrate plan, but no separate pancreas. However, in the wall of the 



Fig. 55. Lake lamprey attached to a bony fish, which also shows the scars of the 

 attacks of other lampreys. (After Gage.) 



anterior part of the intestine there are large patches of cells that 

 resemble those of the acini of the pancreas of higher forms and con- 

 tain secretory granules. Barrington has shown that extracts of this 

 region have a high proteolytic power, the enzyme being of the tryptic 

 type, with its optimum between pH 7-5 and 7-8. Some of this tissue is 

 collected in the walls of short diverticula, reaching forwards. The 

 situation is therefore essentially similar to that found in amphioxus, 

 and we may regard these patches of zymogen cells, or the diverticula, 

 as the forerunners of the exocrine portions of the pancreas. In the 

 lampreys the endocrine portion, not yet identified in amphioxus, also 

 appears. Around the junction of the fore-gut and intestine are groups 

 of follicles that do not communicate with the lumen of the intestine. 

 These 'follicles of Langerhans' were, appropriately enough, first seen 

 by the discoverer of the islets in higher forms, and Barrington has 

 now shown that following destruction of this tissue by cautery there 

 is a rise in blood-sugar. Moreover, after injection of glucose, vacuola- 

 tion of the cells occurs. We may safely conclude that these cells are 

 involved in carbohydrate metabolism, but only one type of cell is 

 present. 



