56 EVOLUTION UE LIFE. 
class of fishes of which the Amphioxus is the representa- 
tive, then, as Membranous Fishes. The first step in com- 
plexity of structure is presented by the simplest of the 
gristly (cartilaginous) fishes, Lamprey. They possess a 
eristly skull, with brain, etc., but there is no lower jaw 
attached to the skull, their mouth being of the sucking 
kind (Cyclostomi). There are no traces of limbs as yet; 
but the sucking fishes have a distinct heart, differing 
from the Amphioxus, wherein we find only slight dila- 
tations of the blood-vessels. The Myxine, or Hag-fish, and 
the Petromyzon, or Lamprey, are representatives of this 
order. In the Chimera we find a lower jaw, but its sus- 
pensorium is still immovable. It furnishes the transition 
from the Lamprey kind to the Sharks. (Fig. 55.) The 
Sharks and Rays (Devil-fish) are still gristly in structure, 
but their jaws are very freely movable, and furnished with 
numerous teeth, which are very characteristic in the differ- 
ent kinds. These teeth are found fossil in great numbers in 
the early rocks, and prove that the gristly fishes were among 
the first Vertebrates that appeared in the seas. The Sharks 
possess two pairs of fins, and their intestine is furnished 
with valves arranged in a spiral or transversely. We come 
next to a class of fishes known as Ganoids, that is, shining. 
In some of these, as in the Sturgeon (Fig. 56), we have 
the backbone still gristly, while in others, as in the Gar- 
pike, it is bony. The outer part of the body is covered 
either with shiny plates (Placoganoids), as in the Coccos- 
teus, Sturgeon, or with shiny scales (Lepidoganoids), as in 
the Gar-pike. It is by means of these shiny plates and 
scales, as well as the whole fish, found in great: profusion, 
well preserved in the early rocks, that we know that the 
Ganoids are very old fish, and that they existed in great 
numbers in the early ages of the earth; whereas at the 
present day the Ganoids are represented only by half a 
dozen kinds, the Sturgeon, Gar-pike, Polypterus, etc. The 
