VERTEBRATA : FISHES. 481 



of the piston-like toothed tongue. 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- 

 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. Sometimes the oral cavity 

 is strengthened by a basket-shaped cartilaginous apparatus, 

 and sometimes a similar apparatus supports the gill-sacs. The 

 alimentary canal is simple and straight, the liver not sac-like, 

 but of its ordinary form, and the kidneys distinct and well 

 developed. 



The Marsipobranchii are peculiar amongst Vertebrate ani- 

 mals in possessing only one median nasal sac, opening on the 

 exterior of the head by a single unpaired nostril. The Hag- 

 fishes further differ from all the members of the class, except 

 the Mud-fishes (Dipnoi} in the fact that the nasal cavity com- 

 municates behind with the pharynx. In the Lampreys, on the 

 other hand, the nasal sac is closed posteriorly. 



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. The gills, namely, are in 

 the form of fixed pouches, instead of being free vascular struc- 

 tures contained in a common chamber, opening externally by 

 a gill-slit, as in the typical Bony Fishes. In the Hag-fishes 

 there are six of these branchial sacs on each side of the oeso- 

 phagus (fig. 260, C). The water is admitted to the gullet (g) 

 by a special aperture situated on the ventral surface, whence it 

 passes into the branchial pouches by six apertures on each 

 side. Having passed over the complicated and highly vascular 

 interior of the branchial sacs, the water escapes by a corre- 

 sponding series of tubes opening into a common canal (c] on 

 each side, and these canals finally discharge the effete water by 

 two apertures situated on the ventral surface behind the head 

 (/i, //). In the Lampreys the gills have the same fixed and 

 pouch-like arrangement, but there are some marked differences 

 from the above. The water is admitted from the gullet to seven 

 branchial pouches on each side, but the mode of admission 

 is by means of two special canals which lie beneath the oeso- 

 phagus on each side, communicating each by its own aperture 

 with the mouth in front, terminating blindly behind, and send- 

 ing off a branch to each pouch. The effete water, also, escapes 

 by a special tube to each sac, so that there are seven branchial 



2 H 



