STURGEONS. 
293 
On the other hand, the scale of organization 
descends to a lower point in the Cartilaginous 
than any reached in the Osseous Orders. In the 
extreme forms the skeleton becomes obsolete ; 
the spine is no longer divided into vertehrce^ but 
is reduced to a soft, flexible, transparent, and 
slender column or thread of cartilage ; and the 
animals can only with the utmost difficulty be 
distinguished from Worms. 
Thus it seems probable that in a truly natural 
arrangement, as the illustrious Cuvier has sug- 
gested, the Cartilaginei ought not to be placed 
either above or below other Fishes, but rather 
as forming a parallel series, or Sub-Class, as the 
Marsupialia form a series of Mammalia, paral- 
lel with the Placentalia. 
The Cartilaginous Fishes are not very numer- 
ous, when compared with the other Orders : they 
are, however, widely scattered, some of them 
being found in all seas, from the equator to the 
seas surrounding either pole. They are almost 
exclusively marine ; the Sturgeons and the Lam- 
preys, however, are exceptions, inhabiting rivers. 
Five Families are included in this Order, named, 
respectively, Acipenserid^e, Chimceradce ^ Squalidce^ 
Raiadce^ and Petromyzonidcd^ 
Family I. AciPENSERiDiE. 
{Sturgeons 
The Sturgeons have their gills free, like other 
fishes, with a single opening, which is compa- 
ratively wide, and protected by a large, oval, 
radiated plate, as a gill-cover ; there are no gill- 
