SENSES AND NERVES I27 



the body of some Protozoa that swim by means of a flagellum ; 

 it is generally found in those kinds that possess one or more 

 granules containing chlorophyll. The chlorophyll enables 

 them to absorb the energy from sunlight for the manufacture 

 of food from carbon dioxide dissolved in the water, in the 

 same way that plants carry on their photosynthesis. The 

 substances in the spot of red colour are changed chemically 

 by the action of light, and as a result the animal swims to- 

 wards the light which will give the greatest amount of 

 energy for the food manufacturing process. The speck of 

 colour, or eye-spot, is the first example of a special sense 

 organelle found in the least complex of the animal kingdom. 

 It does not enable the animal to see, for it forms no image, 

 and even if it did the animal has no brain with which it 

 could interpret the image, but it is a part of the animal 

 specialized to react to light, a photoreceptor. 



The more complex animals are usually larger, and differ 

 from the Protozoa in that their bodies do not consist of a 

 single unit of living matter. The protoplasm is divided into 

 thousands or millions of separate units, each in many ways 

 similar to an Amoeba, stuck together so as to build up the 

 body. Such animals with many-celled bodies are distin- 

 guished from the Protozoa by the name Metazoa, 



In the Metazoa each cell, while still performing the basic 

 functions, is additionally specialized for one or more par- 

 ticular functions. The comparison with people living in a 

 village is obvious ; the butcher does not bake his own bread, 

 nor the baker make his own clothes, and each inhabitant 

 has his own special trade from which all benefit. 



In the animal body, however, the cells depend upon each 

 other much more closely, and are merely components of the 

 whole, for they have no individual independent existence. 

 Cells of one kind are generally massed together to form 

 special tissues such as muscle, fat, glands, or nerves. Com- 

 binations of different tissues as functional units form the 

 various organs. In Metazoa, too, only the cells near the 

 surface can receive stimuli from the outside, and some of 

 them are specialized to form sense organs whose sole function 

 is to receive those stimuli. 



The Sea-anemones and their relations, such as jelly-fish and 



