X TELEOSTEAN FISHES 363 



like " Dorado " of the Rio de la Plata and the ferocious Serrasalmo 

 ( " Piranha," " Palometa " ) of South American rivers. 



The Gymnotidae include the most powerful of the electric fishes — the 

 Electric Eel (Gymnotus) of tropical swamps in the basins of the Amazon 

 and Orinoco. A mule touching one of these fish as it wades through a 

 swamp may be brought down instantaneously by the shock. 



The majority of the fresh-water fishes of the Northern Hemisphere 

 belong to the family Cyprinidae — for example the Carp, Dace, Chub, 

 Roach, Minnow, Bream, Loach. Many are vegetarian feeders and as a 

 rule they are not of much value as food for man. 



The family Siluridae corresponds roughly with what are popularly 

 called " Cat-Fish," perhaps from the sensory tentacle-like " barbels " 

 in the neighbourhood of the mouth. There are no cycloid scales, bony 

 tissue being either absent in the skin of the general surface of the body 

 or, at the other extreme, forming large bony plates which articulate 

 together so as to form a rigid armour coat. 



Included in the group is the powerful electric fish Malopterurus of 

 tropical Africa and Egypt, in which the electric organ is unique in being 

 a development of the skin. It is innervated by a special nerve on each 

 side of the body, consisting of a single gigantic nerve-fibre enclosed in 

 a thick insulating sheath (medullary sheath), dividing into an immense 

 number of branches, and originating in a large ganglion-cell situated in 

 the spinal cord near its front end. 



The Siluroids inhabit for the most part fresh water and the group is 

 widely spread over the surface of the earth. 



The Anguillidae have long snake-like bodies. The scales are much 

 reduced in size (Common Eel — Anguilla) or entirely absent (Conger-eel 

 — Conger). The pelvic fins have disappeared. 



Until comparatively modern times the life-history of the eel formed 

 an unsolved puzzle, fully ripe specimens of the common eel being un- 

 known. It is now known that the eel is catadromous, i.e. it descends 

 the rivers into the sea to spawn. As sexual maturity approaches the 

 eels wander down the rivers and having reached the sea continue their 

 migration, in the case of European eels, westward towards the spawning 

 grounds which are situated in the West Atlantic, south of Bermuda. 

 The eggs develop into pelagic larvae differing greatly from the parent 

 eel in appearance, the body being greatly flattened from side to side and 

 much deepened in a dorso-ventral direction. They are of a glassy trans- 

 parency and colourless, the blood even being devoid of haemoglobin. 

 Such larvae were formerly classified together as a genus by themselves 

 and given the name Leptocefhalus. The Leptocephalus larvae drift 



