CHAPTER XV 



THE ROUND-MOUTHED AND CARTILAGINOUS FISHES 



THE Cyclostomata, or Round-mouthed fishes, are a primitive 

 class of vertebrates, divided into two orders, the first containing 

 the Hag-fishes or " Borers " (Myxinoids), and the second the 

 Lampreys (Petromyzonts). 



Cyclostomes are distinguished generally by their eel-like form 

 and smooth, scaleless skin, the round, suctorial mouth, which 

 is furnished with horny teeth but no definitely developed jaws, 

 by the absence of paired fins or girdle, the sac-like gill-pouches, 

 cartilaginous skeleton, and unsegmented notochord. 



The Hag-fishes are the simplest, and probably the most primi- 

 tive, of all craniates. They are curious, long, eel-shaped creatures, 

 with a somewhat blunted, conical snout, at the extremity of which 

 is the characteristic round mouth. Above the mouth is a single 

 nostril, the channel by which the water enters on its way to 

 the gills. Four pairs of barbules or short tentacles surround the 

 mouth, and in the roof of the mouth is a single horny tooth. The 

 Hags also possess a powerful muscular " tongue," which works 

 like a piston, and is provided with two pairs of comb-like tooth- 

 plates. This tongue is used as a rasping organ, and has developed 

 to such an extent that the gill-sacs and the heart have been pushed 

 back abnormally far. 



These curious creatures have a very wide distribution ; they 

 are entirely marine, living in the mud of the sea-bottom at depths 

 ranging from 50 to 350 fathoms, or coiled up at rest among the 

 rocks. They are sedentary in their habits, remaining motionless 

 for long periods, especially during the daytime, but are able to 

 swim rapidly with a graceful, serpentine motion. Down each side 

 of the body is a row of mucus-secreting glands forming a bead-like 

 chain, from which large quantities of slime are ejected. 



The Hag-fishes are particularly troublesome to fishermen, and 



211 



