Education through Nature 



as to expose as much surface as possible. By the 

 simple law of gaseous diffusion oxygen is taken up 

 by the blood and carbon dioxide is given off. In 

 higher animals a substance called haemoglobin exists 

 in the red corpuscles; it has a special affinity for oxygen. 



The same simple principle of construction is found 

 in the more complex respiratory organs trachea of 

 insects; gills of aquatic insects, mollusks, Crustacea, 

 fishes, and amphibians; lungs of land mollusks, am- 

 phibians, reptiles, birds, and mammals. The lungs 

 of land snails and those of spiders are hardly more 

 than simple bags, on the walls of which capillaries 

 are spread. In the spider folds are formed resembling 

 the leaves of a book. Gills are either in the form of 

 lamellae or else in the form of tufts of threads, as in 

 some worms. 



All these forms of breathing organs are well adapted 

 to secure a large surface. In marine forms the respira- 

 tory surface is usually freely exposed; while in air- 

 breathing forms it is enclosed in the body, as the lungs 

 of higher animals, for the purpose of preventing drying 

 of the surface from exposure to the air. 



SENSATION. All living things are more or less sensi- 

 tive to stimuli, such as contact with other bodies, chemi- 

 cal reagents, heat, cold, electricity, etc. The property 

 of irritability belongs to protoplasm. In the protozoa 

 there is no separate nervous system. The first traces 

 of such a system are found perhaps in the hydra, where 

 some of the ectoderm- cells seem especially sensitive 

 to stimuli. In the jellyfishes the nervous system 

 probably forms a network of cells all over the umbrella 

 of the animal. In the starfish and other echinoderms 

 it forms a ring around the mouth, sending off a nerve- 

 cord into each arm. All invertebrates have this 

 nerve-ring encircling the oesophagus. Worms, Crus- 

 tacea, and insects have a ventral chain of ganglia, a 

 pair of ganglia for each segment. In vertebrates a 



