CHAPTER XVIII. 
CARTILAGINOUS FISHES. 
Cartilaginous fishes are generally animals of considerable size tlieir 
structure ranging from ordinary fishes to eels. They are chiefly sea- 
fishes, only a few species being river-fishes. Naturalists divide them 
into two orders; namely, those having the gills free on the outer 
edge (the gilled Chondropierycjems), and those having these organs 
fixed on both edges. The first order comprehend three families^: I. 
Oyclostomata, or Eels, Lampreys, &c., in which the mouth forms a 
sucker; II. Selachians, including Raias and Sharks, in which the 
mouth is furnished with jaws ; III. Sturonia, or Sturgeons, which 
have the gills free. 
I. Oyclostomata. 
The first are characterised by the singular conformation of the 
mouth, which is formed for suction. The body is elongated, naked, 
and viscous, reminding us of serpents in their external' form ; they 
fiave neither pectoral nor ventral fins ; their vertebra is rednced to 
simple cartilaginous rings, scarcely perceptible one from the other, 
laversed by tendons, and covered by a second and more solid series of 
r mgs, which surround the soft cartilaginous spine. Their gills, in 
Place of presenting the comb-like appearance of other fishes, have 
something of the form ol a purse. The lampreys may be considered 
as the type of this family. 
The Lampreys ( Petromyzon ) are cylindrical, with seven gill-openings 
°n each side of the neck, forming two longitudinal lines; mouth round, 
mined with many teeth. The Sea Lamprey, P. marinus (Fig. 347), 
2 k 2 
