240 VEKTEBRATE ANIMALS. 



Myxinoids, however, such a communieatiun does exist. The nasal 

 sac is placed in coniiminication with tlie throat (pharynx) by means 

 of a canal which perforates the palate. A seconil canal leads from 

 the nasal cavity in front to open liy an external aperture (the nostril) 

 on the top of the head behind the mouth. 



Another peculiarity, which is best cnnsidered iu the Lampreys, is 

 to be found iu tlie structure of the respiratory organs, from which 

 the name of the order is derived. When viewed externallj', instead 

 of the .single great "gill-slit," covered 1 13' a "gill-cover," as .seen in 

 the ordiuary Bony Fislies, the side of tlie neck presents seven round 

 holes placed far back in a line on eacli side. These holes are the 

 external apertures of the gills (tig. 172, B), which in these fishes are 

 in the foriu of sacs or pouches, the lining membrane of wdiich is 

 thrown into numerous folds oi' ])laits, over which the In-anchial ves- 

 sels ramify. Internally the sacs communicate with the cavity of 

 the pharynx, by me.'ius of a coumion respiratory tube iuto which 

 they all open. It follows from this arrangement that the gill- 

 pouches on the two sides of the neck communicate freely with one 

 anotlier through the jiharynx. The object of this arrangement is to 

 obviate the necessity for admitting the water to the gills through 

 the mouth, as (jrdinary tishes do. These fishes are iu the habit of 

 fixing themselves to fureigu objects by means of the suctorial 

 mouth; and when in this position, it is, of course, impossible that 

 they can ol)tain the necessary water of respiration through the 

 UKJUth. As the gill-.sacs, however, on the two sides of the neck 

 communicate freely with one another tlu'ough the pharynx, water 

 can readily pass in and out. The gills are not pi-ovided with cilia, 

 but the circulation of water is assisted by a kind of elastic cartilag- 

 inous framework u]>on which the whole respiratory apparatus is 

 supported, anil which acts somewhat like the ribs of the higher 

 Vertebrates. 



In the Hag-fishes, the structure of the respiratory organs is essen- 

 tially the same as in the Laru]ireys ; but the gill-j)ouches on each 

 side are connected with a common tube, each of whicli opens on the 

 ventral siirface of the body by a small aperture placed at a consider- 

 altle distance behind the head (fig. 172, A). 



The Lam|)rey3 are distinguished from the Hag-fishe.s, not only by 

 having separate and laterally placed apertures to the gill-]iouches, 

 but also by the fact that the nasal sac does not ojien beliind into 

 the throiit. Some of them are inhal)it:ints of rivers; but others 

 live in the sea, and, like the grc;it Sea-l,'imprey (Petromj/zon mari- 

 nes}, betake themselves to flesh waters for the purpose of spawning. 

 The young live in livers, and ))ass thi'ough a c\ii'ious metamor])hosis. 



