COMPARATIVE MORPHOLOGY. 203 
development between plants and animals, has, in sub- 
stance, been merely confirmed and illustrated by the 
labours of the half century which has elapsed since its 
promulgation, 
Not only is it true that the mmute structure of the 
crayfish is, in principle, the same as that of any other 
animal, or of any plant, however different it may be in 
detail; but, in all animals (save some exceptional forms) 
above the lowest, the body is similarly composed of 
three layers, ectoderm, mesoderm, and endoderm, dis- 
posed around a central alimentary cavity. The ectoderm 
and the endoderm always retain their epithelial character ; 
while the mesoderm, which is insignificant in the lower 
organisms, becomes, in the higher, far more complicated 
even than it is in the crayfish. 
Moreover, in the whole of the Arthropoda, and the 
whole of the Vertebrata, to say nothing of other groups 
of animals, the body, as in the crayfish, is susceptible 
of distinction into a series of more or less numerous 
segments, composed of homologous parts. In each 
segment these parts are modified according to physio- 
logical requirements; and by the coalescence, segrega- 
tion, and change of relative size and position of the 
segments, well characterized regions of the body are 
marked out. And it is remarkable that precisely the 
same principles are illustrated by the morphology of 
plants. A flower with its whorls of sepals, petals, 
stamens and carpels has the same relation to a stem 
