THE ROUND-MOUTHED FISHES 213 



same time drawn apart, so as to present an almost flat surface. 

 Placing this flat surface against the fish to be eaten, the Hag 

 draws the halves of the tooth-plate together, thus tearing off a 

 portion of the food, and then withdraws it into its mouth. It 

 swallows the food very rapidly, and immediately sticks out the 

 tooth-plate for more." 1 



The eggs of the Hag-fishes are enclosed in a horny, yellow 

 envelope, somewhat resembling a small sausage, about an inch 

 long. At each end are a number of threads ending in anchor- 

 like knobs, and several eggs are usually fastened together in a 

 chain by the entanglement of these hairs. When hatched the 

 young Hag-fishes resemble the adults, and do not pass through 

 a larval state. They attain a length of from 15 inches to nearly 

 2 feet, but their rate of growth is very slow in comparison with 

 many other vertebrates. 



The Hag-fishes are commonly divided into two distinct families : 

 the Myxinida, which includes a single genus only Myxine species 

 of which are found on the coasts of Northern Europe, including 

 the British Isles, off the coasts of Japan and Chili and on the 

 Atlantic seaboard of North America ; and the Bdellostomatidce, 

 of which species occur on the Pacific seaboard of North and South 

 America, on the coasts of New Zealand and at the Cape of Good 

 Hope. The Bdellostomatida? are distinguished by possessing from 

 six to fourteen pairs of gill-pouches, while the Myxinida never 

 have more than six pairs. 



The second order of the Cyclostomata comprises the Petro- 

 myzonts, or Lampreys, which, although they have many points 

 in common with the hags, differ from them in several peculiarities. 

 The characteristic round mouth is surrounded by a curious funnel- 

 shaped sucker, fringed with small cirri and dotted all over inside 

 with numerous horny, hook-like teeth. The single nostril does 

 not open into the roof of the mouth as in the hag-fishes, but is 

 placed far back on the top of the head behind the funnel. The 

 eyes of the Lampreys are well developed, while those of the hags 

 are degenerate. The eggs of the Lamprey are small and thin- 

 shelled, and the embryo hatches out as a larva and undergoes 

 a metamorphosis before the perfect adult form is reached. The 



1 " Contributions to our Knowledge of the Myxinoids" by Julia Wotthington, 

 1905. 



