ORDERS OF FISHES. 239 
lining membrane of which is thrown into numerous folds or 
plaits, over which the branchial vessels ramify (fig. 128, B). In- 
ternally the sacs communicate with the cavity of the pharynx, 
by 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 
through the pharynx. The object of this arrangement is to 
obviate the necessity for admitting the water to the gills through 
the mouth, as ordinary fishes do. These fishes are in the habit 
of fixing themselves to foreign objects by means of the suctorial 
mouth ; and 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 
Fig. 128.—A, Lamprey (Petvomyzon), showing the sucking-mouth and the aper- 
tures of the gill-sacs. B, Diagram to illustrate the structure of the gills in the 
Lampreys. @ Pharynx; 4 Tube leading from the pharynx into one of the 
gill-sacs ; c One of the gill-sacs, showing the lining membrane thrown into 
folds; d@ External opening of the gill-sac. (In reality the gill-sacs do not open 
directly into the pharynx, but into a common respiratory tube which communi- 
cates with the pharynx ; but this is omitted for the sake of clearness.) 
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 cartilaginous framework upon which the whole respira- 
tory apparatus is supported, and which acts somewhat like the 
ribs of the higher Vertebrates. 
The nasal cavities of the Lampreys, unlike those of the Myxi- 
noids, are closed behind, and do not communicate with the 
throat. Some of the Lampreys are permanently inhabitants of 
rivers, but the great sea-lamprey (Petromyzon marinus) only 
quits the salt water and betakes itself to fresh in order to 
deposit its eggs. 
