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 great shield-like shell, called technically the “carapace,” ending in front in a sharp spine, on either side of which are the curious compound eyes, set upon the ends of stout movable stalks.  Behind these, on the under side of the body, are two pairs of long feelers, or antennae, followed by six pairs of jaws folded against one another over the mouth, and five pairs of legs, the foremost of these being the great pinchers, or claws, of the lobster.