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 minute 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 cavit}^ 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 Vertehrata, 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 



