362 MANUAL OF ZOOLOGY SECT, xn 



well as its external opening through a small gill-slit. In 

 Myxine, on the other hand, though each pouch has a 

 separate internal communication with the pharynx, the 

 tubes leading outwards from the gill pouches of each side 

 all join to form a common tube, which opens on the 

 exterior by the single gill-slit. 



The other systems of organs are not so remarkable. The 

 alimentary canal, the heart and the brain, are not widely 

 different from those of the true Fishes. A peculiar feature is 

 that there is only a single nasal cavity (na. c.} (opening by 

 the single nasal aperture already referred to), instead of the 

 pair developed in all other Craniates ; in Myxine its cavity 

 communicates by a passage with the cavity of the mouth. In 

 the Lamprey in addition to paired eyes having the typical 

 vertebrate structure, there is connected with a lobe in the 

 roof of the fore-brain, a median or pineal eye of simpler 

 structure and imperfectly understood function. 



Lampreys live mainly in rivers and estuaries. Their 

 food consists chiefly of small aquatic animals, such as worms, 

 small Crustaceans, &c. ; but they also sometimes attach 

 themselves to the bodies of Fishes by means of the sucker- 

 like buccal funnel, and rasp off portions of the flesh with the 

 horny teeth of the tongue. Myxine actually bores its way 

 into the interior of the bodies of large Fishes, such as the 

 Cod, consuming the flesh in its passage, and thus becomes 

 for a time an internal parasite almost the only example 

 among the Vertebrata of such a condition. In the free 

 state Myxine usually lies buried in the sand, only the 

 anterior end, with the nasal aperture, projecting on the sur- 

 face. By means of the passage leading from the nasal sac 

 to the mouth, water passes in and out through the nasal 

 aperture, and the process of respiration is carried on while 

 the animal remains almost completely hidden. 



