240 VERTEBRATE ANIMALS. 



Mj'xinoids, however, sucli a communication does exist. The nasal 

 sac is placed in communication with the throat (pharynx) by means 

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

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

 on the top of the head behind the moutli. 



Another peculiarity, which is best considered in the Lampreys, is 

 to be found in the structure of the respiratory organs, from which 

 the name of the order is derived. When viewed externally, instead 

 of the single great " gill-slit," covered by a " gill-cover," as seen in 

 the ordinary Bony Fishes, the side of the neck jjresents seven round 

 holes ])laced far back in a line on each side. These holes are the 

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

 in the foini of sacs or pouches, the lining membrane of which is 

 thrown into numerous folds or plaits, over which the branchial ves- 

 sels I'amify. Internally the sacs communicate with the cavity of 

 the pharynx, Ijy means of a common respiratory tube into which 

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

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

 another tlirough the pharynx. The object of this arrangement is to 

 obviate the necessity for admitting the water to the gills through 

 the niiiuth, as oidiuary fishes do. These fishes are in the habit of 

 fixing themselves to foreign objects by means of the suctorial 

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

 they can obtain the necessary water of respiration through the 

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

 communicate freely with one another through the pharynx, water 

 can readily pass in and out. The gills are not provided with cilia, 

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

 inous framework upon which the whole respiratory apparatus is 

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

 Vertebr.-iti-'S. 



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

 tially the same as in the Lamprej-s ; bxit the gill-pouches on each 

 side are connected with a common tidje, each of which opens on the 

 ventral surface of the body by a small ajierture placed at a consider- 

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



The Lami>rey3 are distinguished from the Hag-fishes, not only by 

 having se]>arate and laterally placed apertures to the gill-pouches, 

 but also Ijy the fact that the nasal sac does not open beliind into 

 the throat. .Siuiie of them are inhabitants of rivers; but others 

 live in the sea, and, like the great Sea-lamprey {Petriimi/zon mari- 

 niis), betake themselves to fresh waters for the purpose of spawning. 

 The young live in rivers, and pass through a curious metamorphosis. 



