The marks of a Vertebrate animal are very easy to remember.
It must have this wonderfully jointed backbone, and also what is called the
skeleton, which is a framework of bone.
A spinal cord (from which this division of animals is sometimes called the
"Chordate").
Four limbs, and red blood.
In these respects all the animals which belong to this division are alike,
though in general appearance they may be as unlike each other as a horse is
unlike a bird, or a crocodile unlike a herring.
Few things in nature are more wonderful than the way in which this
Vertebrate plan has been fitted to animals differing from each other in all
other respects.
Now let us look at the marks of an Invertebrate or Inchordate animal.
It has _no backbone_, and instead of a bony framework _within_, to support
the soft parts of its body, it generally has a hard shell, or thickened
skin _outside_, to protect the softer inner parts.
It has _no red blood_.
Now, just as plants have been arranged in different classes, so animals are
classified according to the various plans upon which they have been
formed. So, besides the two great divisions of the Vertebrates and the
Invertebrates, the latter have been classed as--
(a) _Radiata_, or Rayed Animals--those whose parts all radiate from a
common centre--such as the starfish, red-coral, sea-anemone.
(b) _Mollusca_, or Soft-bodied Animals, protected by shells--such as
snails, oysters, limpets.
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