modifications--beneath and through which a common plan of formation is
discernible. But if I look at the same part physiologically, I see
that it is a most beautifully constructed organ of locomotion, by means
of which the animal can swiftly propel itself either backwards or
forwards.

But how is this remarkable propulsive machine made to perform its
functions? If I were suddenly to kill one of these animals and to take
out all the soft parts, I should find the shell to be perfectly inert,
to have no more power of moving itself than is possessed by the
machinery of a mill when disconnected from its steam-engine or
water-wheel. But if I were to open it, and take out the viscera only,
leaving the white flesh, I should perceive that the lobster could bend
and extend its tail as well as before. If I were to cut off the tail, I
should cease to find any spontaneous motion in it; but on pinching any
portion of the flesh, I should observe that it underwent a very curious
change--each fibre becoming shorter and thicker. By this act of
contraction, as it is termed, the parts to which the ends of the fibre
are attached are, of course, approximated; and according to the
relations of their points of attachment to the centres of motions of
the different rings, the bending or the extension of the tail results.
Close observation of the newly-opened lobster would soon show that all
its movements are due to the same cause--the shortening and thickening
of these fleshy fibres, which are technically called muscles.

Here, then, is a capital fact. The movements of the lobster are due to
muscular contractility. But why does a muscle contract at one time and
not at another? Why does one whole group of muscles contract when the
lobster wishes to extend his tail, and another group when he desires to
bend it? What is it originates, directs, and controls the motive