Ko EVOLUTION OF LIFE 
Anatomy, Embryology, and Geology, when taken together, 
make it most probable that the Tree of the Mollusca is 
such as represented. 
With the Mollusca we leave the Invertebrata, or animals 
without a backbone, and turn to the Vertebrata. 
VERTEBRATA. 
This division includes—tst, the Fishes ; 2d, the Batrachia 
(frogs, etc.) ; 3d, the Reptiles (snakes, etc.) ; 4th, the Birds; 
sth, the Mammals (animals suckling their young). They 
all possess a backbone, rudimentary in some fishes. This 
backbone is composed of separate bony pieces known as 
vertebrae; hence the name of Vertebrata given to the five 
classes just mentioned. Running through this backbone, 
spine, or vertebral column, as it is differently called, is seen 
a nervous cord which expands 
the marrow or spinal cord, 
into the brain, which is inclosed by the skull-bones. Such , 
a structure is never seen in a star-fish, insect, or mollusk. 
The Vertebrata never possess more than two pairs of 
limbs. The muscles moving these limbs are attached to 
bones, which, together with the skull and backbone, form 
the skeleton. The skeleton is the most characteristic 
feature of the Vertebrata, and nothing like it is met with in 
the Invertebrata, called also Evertebrata, that is, without 
vertebra. There are apparent exceptions, such as the wing 
of an insect, among the Invertebrates, which is used like 
the wing of a bird; but the wing of the insect is only an 
expansion of the skin, whereas the wing of a bird is always 
supported by bone. The wing of the insect and that of the 
bird are said to be analogous, because they are used for the 
same purpose; they are not homologous, because they have 
not the same structure. The jaws of a Vertebrate are always 
parts of the head, never, as in many of the Crabs, modifica- 
tions of the anterior limbs. There are found at different 
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