exhibiting the same broad peculiarities.

I am sure that you cannot have followed me even in this extremely
elementary exposition of the structural relations of animals, without
seeing what I have been driving at all through, which is, to show you
that, step by step, naturalists have come to the idea of a unity of
plan, or conformity of construction, among animals which appeared at
first sight to be extremely dissimilar.

And here you have evidence of such a unity of plan among all the
animals which have backbones, and which we technically call
"Vertebrata". But there are multitudes of other animals, such as
crabs, lobsters, spiders, and so on, which we term "Annulosa". In
these I could not point out to you the parts that correspond with those
of the Horse,--the backbone, for instance,--as they are constructed
upon a very different principle, which is also common to all of them;
that is to say, the Lobster, the Spider, and the Centipede, have a
common plan running through their whole arrangement, in just the same
way that the Horse, the Dog, and the Porpoise assimilate to each other.

Yet other creatures--whelks, cuttlefishes, oysters, snails, and all
their tribe ("Mollusca")--resemble one another in the same way, but
differ from both "Vertebrata" and "Annulosa"; and the like is true of
the animals called "Coelenterata" (Polypes) and "Protozoa" (animalcules
and sponges).

Now, by pursuing this sort of comparison, naturalists have arrived at
the conviction that there are,--some think five, and some seven,--but
certainly not more than the latter number--and perhaps it is simpler to
assume five--distinct plans or constructions in the whole of the animal