CHAPTER VII 



CRUSTACEA AND MOLLUSCS 



Economy of nature. The chemical constituents of bodies. Chemical 

 combinations. The elements. The albuminoids. Biogens as 

 constituents of living matter. Vital phenomena and apparent 

 death. Metabolism. Structure of the living substance. Plants the 

 foundation of life. Order of sustenance in nature. Flesh-eating 

 is more natural to the animal than plant-diet. Are all variations 

 useful to animals ? Value of selection. Origin of the shells of 

 snails. Change of functions. Development of the crab. Why the 

 embryonic development of an animal reproduces its racial history. 

 Reconstruction of embryogenesis. Uncertainty of the biogenetic 

 law. Parthenogenesis, the development of unfertilised eggs. 

 Significance of the germ-cells. Significance of sexual reproduction. 

 Amphimixis. Plural variations. 



IN every pond there are living at a certain depth 

 innumerable tiny creatures up to the size of a pin's head. 

 They pass up and down unceasingly, and travel back- 

 wards and forwards through the water. Every hour 

 swarms of them are devoured by their countless enemies, 

 yet we see no sign whatever of a diminution of their 

 numbers. The heat of summer broods over the pond, 

 the vapour rises to the clouds, aquatic plants come to the 

 surface and wither in the heat ; in a short time there is, 

 instead of the pond, a dry earth-pit covered with animal 

 remains. In the autumn the rain sets in, the ground is 

 moistened, little pools are formed, and soon the rich 



colours of autumn are reflected on the surface of the 



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