THE CRUSTACEA. 279 



prawn-lilie animals, which, from the fact that they all 

 have their eyes set upon movable stalks, are termed the 

 Poclophtlialmia, 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 form.s 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 Pocloph- 

 tlialmia 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 



