THE CRUSTACEA. 279 
prawn-like animals, which, from the fact that they all 
have their eyes set upon movable stalks, are termed the 
Podophthalmia, or stalk-eyed Crustacea; and by argu- 
ments of similar force to prove that they are all modifica- 
tions of the same common plan. Not only so, but the 
sand-hoppers of the sea-shore, the wood-lice of the land, 
and the water-fleas or the monoculi of the ponds, nay, 
even such remote forms as the barnacles which adhere to 
floating wood, and the acorn shells which crowd every inch 
of rock on many of our coasts, reveal the same funda- 
mental organization. Further than this, the spiders 
and the scorpions, the millipedes and the centipedes, and 
the multitudinous legions of the insect world, show us, 
amid infinite diversity of detail, nothing which is new in 
principle to any one who has mastered the morphology 
of the crayfish. 
Given a body divided into somites, each with a pair 
of appendages; and given the power to modify those 
somites and their appendages in strict accordance with 
the principles by which the common plan of the Podoph- 
thalmia is modified in the actually existing members of 
that order; and the whole of the Arthropoda, which 
probably make up two-thirds of the animal world, might 
readily be educed from one primitive form. 
And this conclusion is not merely speculative. As a 
matter of observation, though the Arthropoda are not all 
evolved from one primitive form, in one sense of the 
words, yet they are in another. For each can be traced 
