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. (The members of this family are numerous indeed).
(c) Annulosa, or Ringed Animals—those whose bodies are composed of many parts, jointed together—such as crabs, spiders, bees, ants, centipedes, shrimps, and many more; for this great family has relations among all the insect tribes.
It is very beautiful to see that God has formed His creatures on such different plans, and though we shall be able to say very little about them, I hope you will by-and-by study Natural History, and learn more and more of His care in fitting each for the life it has to live. But remember that all these types of animals, the Radiates, Molluscs, Articulates (as the members of the "ringed" family are sometimes sailed), existed in the most ancient times: they lived side by side, as it were, and were not, as some philosophers would have us believe, derived from each other. Each was "after its kind," and each species remains; animals may alter from changes in their way of life, but there is no passing from one kind to another.
Now I think you will be interested to hear that in the Stone Book, some of the most ancient "letters" are formed from creatures belonging to the Invertebrate Group. We were speaking just now of the white clay brought up from the depths of the Atlantic Ocean by the sounding line. The microscope shows that it consists of the imperishable part of creatures, tinier than any you can imagine, which had the power when living of extracting from the sea-water—as I told you is the way of the corals—the lime which formed their outer coat, or skeleton. These busy workers lived their little day, and then as they died, the shell-like coverings sank to the bottom of the sea, forming, as ages passed, thick beds of chalk, such as that of which the white cliffs of Dover are built up.