ORDEKS OF FISHES. 



217 



vessels ramify (Fig. 106, B). Internally the sacs communi- 

 cate 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 phar- 

 ynx. The object of this arrangement is to obviate the ne- 

 cessity for admitting the water to the gUls through the mouth, 



Fig. 106. — A, Lamproy {P0tromi/son\ stowing the suckin^-moutli and the apertures of 

 the gill-sacs. B, Diagram to illustrate the structure of the gills in the Lampreys, a 

 Pharynx; & Tube leading from the pharynx into one of the gill-sacs; c One of the giU- 

 saes, 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 communicates with the pharynx ; but this is omitted for the 

 sake of clearness.) 



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 

 neck communicate freely with one another through the phar- 

 ynx, 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 respiratory apparatus is supported, and which acts some- 

 what like the ribs of the higher Vertebrates. 



The^ nasal cavities of the Lampreys, unlike those of the 

 Myxinoids, are closed behind, and do not communicate with 

 the throat. Some of the Lampreys are permanently inhabit- 

 ants of rivers, but the great sea-lamprey (Petromyzon mari- 

 nus) only quits the salt water and betakes itself to fresh in 

 order to deposit its eggs. 



Oedbe in. Tbleostei (Gr. tehios, perfect; and osteon, 

 bone). — The fishes comprised in this order, as implied in their 

 name, have a well-ossified or bony skeleton, and they are com- 



