356 MANUAL OF ZOOLOGY. 



round the hinder extremity of the body. The skeleton re- 

 mains throughout life in a cartilaginous condition, the chorda 

 dorsalis is persistent, and the only traces of bodies of vertebrae 

 are found in hardly perceptible rings of osseous matter de- 

 veloped in the sheath of the notochord. The neural arches of 

 the vertebrae, enclosing the spinal cord, are only represented 

 by cartilaginous prolongations. The mouth in the Hag-fish 

 (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 larger 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 previ- 

 ously attached itself by its sucker-like mouth. In the Lam- 

 preys 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 

 structures 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 (Lepidosiren\ 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 

 which is in reality an unpaired nostril. In the Lampreys, on 

 the other hand, the single nasal sac has the same structure as 

 in the typical fishes that is to say, it is closed behind, and 

 does not communicate in any way with the cavity of the 

 mouth. 



Another very remarkable point in the Hag-fishes and Lam- 

 preys is to be found in the structure of the gills, from which 

 the name of the order is derived. In the Lampreys, in place 



