ROUND-MOUTHED FISHES 303 



Eel (Gymnotus electricus), which inhabits the fresh waters of tropical 

 South America. It is closely related to our common eel, and by means 

 of its extremely painful shock can stun even human beings. 



The Saw-Fishes (Pristis) occupy an intermediate position between 

 the sharks and rays. They take their name from a long projection of 

 the snout, each side of which is furnished with a row of sharp teeth, 

 giving it the appearance of a saw. This organ forms a formidable 

 weapon of attack and defence. 



ORDER IV.: ROUND-MOUTHED FISHES (CYCLOS- 



TOMI). 



SKELETON cartilaginous. The vertebral column is in the form of an 

 unsegmented notochord ; segmentation indicated by cartilaginous verte- 

 bral arches. Six or seven pairs of sac- like gills ; mouth circular ; a single 

 (unpaired) olfactory (nasal) pit. 



The River-Lamprey (Petromyzonfluviatilis). 

 (Length up to 20 inches.) 



The lamprey inhabits the littoral waters of Europe, North America 

 and Japan. It is a bottom fish, and as such has its upper side dark- 

 coloured (olive green to brown). (Compare with tench.) At the spawning 

 season the fish leaves the sea and migrates up the rivers in order to 

 deposit its eggs in sandy spots near the bank. 

 (The young, called ammocoetes, have the form of 

 worm-like larvae, and only reach the parent stage 

 after several transformations.) In a feeble cur- 

 rent the lamprey progresses by undulations of its 

 eel-like body, which is furnished with only two 

 dorsal fins and a caudal fin (see eel). In a strong 

 current, on the other hand, progress is effected 

 by leaps, the fish propelling itself forwards with 

 a jerk, and attaching itself firmly by means of its 

 peculiarly constructed mouth (suctorial mouth) 

 to a stone or other object. The mouth when ANTERIOR PoRTION OF BO DY 

 open has the form of a circular disc. In its OF THE LAMPREY (MOUTH 

 funnel-shaped bottom lies a piston-like organ, * EN) ' 



,, TTT , *,. . Z., Tongue ; K., the three first 



the so-called tongue. When this organ is re- gill openings, 



tracted, a vacuum is produced in the cavity of 



the mouth, in virtue of which the animal can attach itself to any object, 

 as firmly as a cupping-glass applied to the human body (compare with 



