On the Study of Zoology | Page 5

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.
It looks, at first, a little hopeless to attempt to find in this complex mass
a series of rings, each with its pair of appendages, such as I have shown
you in the abdomen, and yet it is not difficult to demonstrate their
existence. Strip off the legs, and you will find that each pair is attached
to a very definite segment of the under wall of the body; but these
segments, instead of being the lower parts of free rings, as in the tail,
are such parts of rings which are all solidly united and bound together;
and the like is true of the jaws, the feelers, and the eye-stalks, every
pair of which is borne upon its own special segment. Thus the
conclusion is gradually forced upon us, that the body of the lobster is
composed of as many rings as there are pairs of appendages, namely,
twenty in all, but that the six hindmost rings remain free and movable,
while the fourteen front rings become firmly soldered together, their
backs forming one continuous shield--the carapace.
Unity of plan, diversity in execution, is the lesson taught by the study
of the rings of the body, and the same instruction is given still more
emphatically by the appendages. If I examine the outermost jaw I find
it consists of three distinct portions, an inner, a middle, and an outer,
mounted upon a common stem; and if I compare this jaw with the legs
behind it, or the jaws in front of it, I find it quite easy to see, that, in the
legs, it is the part of the appendage which corresponds with the inner
division, which becomes modified into what we know familiarly as the
"leg," while the middle division disappears, and the outer division is
hidden under the carapace. Nor is it more difficult to discern that, in the

appendages of the tail, the middle division appears again and the outer
vanishes; while, on the other hand, in the foremost jaw, the so-called
mandible, the inner division only is left; and, in the same way, the parts
of the feelers and of the eye-stalks can be identified with those of the
legs and jaws.
But whither does all this tend? To the very remarkable conclusion that
a unity of plan, of the same kind as that discoverable in the tail or
abdomen of the lobster, pervades the whole organization of its skeleton,
so that I can return to the diagram representing any one of the rings of
the tail, which I drew upon the board, and by adding a third division to
each appendage, I can use it as a sort of scheme or plan of any ring of
the body. I can give names to all the parts of that figure, and then if I
take any segment of the body of the lobster, I can point out to you
exactly, what modification the general plan has undergone in that
particular segment; what part has remained movable, and what has
become fixed to another; what has been excessively developed and
metamorphosed and what has been suppressed.
But I imagine I hear the question, How is all this to be tested? No doubt
it is a pretty and ingenious way of looking at the structure of any
animal; but is it anything more? Does Nature acknowledge, in any
deeper way, this unity of plan we seem to trace?
The