364 MANUAL OF ZOOLOGY sect. 



composed of cartilaginous processes. This branchial basket, 

 as it is termed, supports the gill-sacs. 



The gill-sacs, of which there are either six or seven pairs, 

 are the organs of respiration, representing the gills of the 

 true fishes. In the lamprey each of these communicates 

 with the exterior by the corresponding gill-slit, and inter- 

 nally opens into a common passage, the respiratory tube 

 which leads in front into the buccal cavity. In Bdellostoma 

 each gill-pouch has its own internal opening through a 

 narrow tube into the pharynx, as well as its external open- 

 ing through a small gill-slit. In Myxine, on the other 

 hand, though each pouch has a separate internal commu- 

 nication 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 sac (opening by the single 

 nasal aperture already referred to) instead of the pair 

 developed in all other Craniates ; in Myxine its cavity com- 

 municates 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, etc. ; 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 makes its way 

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



