bvious facts it presents, inevitably leads us
into all these branches of zoological science.

I have before me a lobster. When I examine it, what appears to be the
most striking character it presents? Why, I observe that this part which
we call the tail of the lobster, is made up of six distinct hard rings
and a seventh terminal piece. If I separate one of the middle rings, say
the third, I find it carries upon its under surface a pair of limbs or
appendages, each of which consists of a stalk and two terminal pieces. So
that I can represent a transverse section of the ring and its appendages
upon the diagram board in this way.

If I now take the fourth ring, I find it has the same structure, and so
have the fifth and the second; so that, in each of these divisions of the
tail, I find parts which correspond with one another, a ring and two
appendages; and in each appendage a stalk and two end pieces. These
corresponding parts are called, in the technical language of anatomy,
"homologous parts." The ring of the third division is the "homologue" of
the ring of the fifth, the appendage of the former is the homologue of
the appendage of the latter. And, as each division exhibits corresponding
parts in corresponding places, we say that all the divisions are
constructed upon the same plan. But now let us consider the sixth
division. It is similar to, and yet different from, the others. The ring
is essentially the same as in the other divisions; but the appendages
look at first as if they were very different; and yet when we regard them
closely, what do we find? A stalk and two terminal divisions, exactly as
in the others, but the stalk is very short and very thick, the terminal
divisions are very broad and flat, and one of them is divided into two
pieces.

I may say, therefore, that the sixth segment is like the others in plan,
but that it is modified in its details.

The first segment is like the others, so far as its ring is concerned,
and though its appendages differ from any of those yet examined in the
simplicity of their structure, parts corresponding with the stem and one
of the divisions of the appendages of the other segments can be readily
discerned in them.

Thus it appears that the lobster's tail is composed of a series of
segments which are fundamentally similar, though each presents peculiar
modifications of the plan common to all. But when I turn to the forepart
of the body I see, at first, nothing but a gr