CHAPTER III 



PETROMYZONTS OR LAMPREYS 



Characters. Habits. Mode of feeding. Life-history. Classification. Re- 

 lationships of Cyclostomes. 



THE lampreys are marked by many interesting peculiar- 

 ities, which show that they are not nearly related to 

 the hags. The mouth is surrounded by a large suc- 

 torial funnel, which is studded internally with numerous horny 

 "teeth". Other similar teeth are borne on the muscular piston 

 or " tongue ". In consequence of the great development of the 

 sucker, the nostril or naso-pituitary aperture is far back on the 

 top of the head. The canal does not, however, open internally 

 on the roof of the mouth as it does in the hag. There are seven 

 pairs of gill-sacs which open directly to the exterior, and, in 

 the adult, indirectly into the gullet. In the larva the gill-sacs 

 open, as usual, into the pharynx, but at the metamorphosis the 

 part of the gut which receives them is converted into a sub- 

 cesophageal respiratory tube and a new anterior oesophagus is 

 developed. There is an elaborate branchial basket supporting 

 the gill-sacs, the " tongue," and even the heart. Lampreys 

 have normal eyes ; the ear has two (instead of the usual three) 

 semicircular canals ; there is a well-developed lateral line 

 system of sensory structures (of which there is just a trace in 

 the head region of the Californian hag) ; the dorsal and ventral 

 roots of the spinal nerves do not unite ; the brain is poorly de- 

 veloped. There is a fairly well-developed cartilaginous skull. 

 In the intestine there is a hint of the spiral fold, which is large 

 in gristly fishes, serving to increase the digestive absorptive 

 surface. The eggs are small and thin-shelled. As there is re- 

 latively little yolk, the segmentation is total. The embryo 

 develops into a larva which is in many respects different from 

 the adult. There is a marked metamorphosis. Lampreys are 

 29 449 



