MARSIPOBRANCHII. 



403 



(Myxine) is of a very remarkable character, and enables it to 

 lead a very peculiar mode of life. It is usually found, namely, 

 embedded in the interior of some other large fish into which 

 it has succeeded in penetrating by means of its singular dental 

 apparatus. The mouth is sucker-like, destitute of jaws, but 

 provided with tactile filaments or cirri. In the centre of the 

 palate is fixed a single, large, recurved fang, which is firmly 

 attached to the under surface of the cranium. The sides of 

 this fang are strongly serrated, and it is by means of this that 

 the Hag-fish bores its way into its victim, having previously 

 attached itself by its sucker-like mouth. In the Lampreys the 

 mouth has also the form of a circular cup or sucker, and is 

 also destitute of jaws; but in addition to the palatine fang of 

 the Myxine, the margins of the lips bear a number of horny 

 processes, which are not really true teeth, but are hard struc- 



Fig. 170. A, Lamprey (Petrjmyzon), showing the sucking-mouth and the apertures 

 of the gill-sacs. B, Diagram to illustrate the structure of the gills in the Lamprey; 

 a Pharynx ; b Tube leading from the pharynx into one of the gill-sacs ; cOue 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 is omitted for the sake of clearness.) 



tures developed in the labial mucous membrane. The tongue, 

 also, is armed with serrated teeth, and acts as a kind of piston ; 

 so that the Lampreys are in this manner enabled to attach 

 themselves firmly to solid objects. 



A very remarkable peculiarity in the Hag-fishes, and one 

 very necessary to remember, is found in the structure of the 

 nasal sacs. In all fishes, namely, except these and the Mud- 

 fishes (Lepidosireii), the nasal sacs are closed behind, and do 

 not open posteriorly into the throat. In the Myxinoids, how- 

 ever, such a communication exists, and the nasal sac for 

 there is only one is placed in communication with the cavity 

 of the mouth by means of a canal which perforates the palate. 

 In front the nasal cavity communicates with the external me- 

 dium by a second tube, which opens on the top of the head 

 by a single aperture, which is often called the " spiracle," and 



