2 BRITISH FRESHWATER FISHES 



like a piston, rasping off the flesh of the fishes on 

 which these animals prey ; further characteristic 

 peculiarities of the group are the presence of only 

 a single median nasal opening and the structure of 

 the gills, which are parallel vascular laminar ridges 

 on the inner walls of a paired series of well-separated 

 muscular pouches, communicating internally with the 

 pharynx and opening either directly or by means of 

 ducts to the exterior. 



Water is often both taken into and expelled from 

 the branchial sacs through their external apertures; 

 this is accomplished by the alternate expansion and 

 contraction of their muscular walls, and the branchial 

 skeleton, which is well developed only when the 

 sacs are large, lies external to them, and is nearly 

 rigid, serving to prevent the collapse of the 

 branchial region ; this method of respiration is no 

 doubt due to the parasitic habits of the Marsipobranchs, 

 for when they are attached to other fishes they 

 cannot take water through the mouth into the 

 pharynx, and by contracting the latter expel it through 

 the gill-openings, which is the normal method of 

 respiration in other fishes. 



The Hag-fishes are exclusively marine, and many 

 of them occur in deep water; they are in most 

 respects more degenerate than the Lampreys ; they 

 bore right into the fishes which they attack, and 

 devour them until practically nothing is left but 

 skin and bone. 



The Lampreys are found on the coasts and 

 in the rivers of all temperate regions ; there are 

 about twenty species, most of which spend part 

 of their life in the sea, whilst all appear to spawn 

 in fresh water. In them the mouth is surrounded 



