76 ELECTRIC EEL. ORDER CYCLOSTOMI. 



electric powers of the Gymnotus have been already mentioned 

 ( 560) ; the other species of the family, of which a considerable 

 number inhabit the fresh waters of South America, appear to 

 be destitute of any electrical properties. The last family is 

 that of the SYMBRANCHUXE, in which the gill-passages unite, so 

 as to open externally by a single orifice on the lower surface of 

 the neck ; this is sometimes divided by a longitudinal partition. 

 Like the Gymnotidae, they are inhabitants of the fresh waters, 

 and are confined to tropical climates. With these we close the 

 long series of the bony Fishes. 



ORDER IV. CYCLOSTOMI. . 



639. The Fishes of this Order exhibit a very imperfect 

 structure of the skeleton. So far are they from having a jointed 

 vertebral column, that this is replaced, in the highest among 

 them, by a sort of cylinder of cartilage, which represents the 

 bodies of the vertebrae, but which does not show any definite 

 division into segments. There are no ribs, nor are there either 

 pectoral or ventral fins ; there is, in some, however, a kind of 

 fin beneath the tail, but this has no rays. The body is usually 

 elongated, and nearly cylindrical ; and terminated by a circular 

 mouth adapted for sucking. The branchial organs form little 

 sacs, on the walls of which the blood-vessels ramify. 



640. In the PETROMYZONID^, or Lampreys, there are seven 

 gill-openings on each side ; there are strong teeth in the ring of 

 the mouth ; and the inner part of the disc, which may be con- 

 sidered as the lip, is also beset with hard tooth-like tubercles. 



FIG. 386 LAMPREY. 



The tongue, which moves backwards and forwards like a piston. 



