52 ZOOLOGY. 



of certain proteid substances, one of which haemoglobin is notable for 

 its affinity for oxygen. The degeneracy may go to the extent of the entire 

 loss of the nucleus, as in the mammals. The colorless cells have the 

 power of independent motion such as is found in the amoeba, and may ingest 

 solid particles of food. The body-fluids of the invertebrates contain as a 

 rule only colorless corpuscles, and are therefore more like the lymph of 

 the vertebrates. When their blood is colored it is usually from pigment 

 in the plasma or fluid portion of the blood. In addition to the cells the 

 blood carries a rich supply of proteid and other substances for use in the 

 tissues, and of waste products in process of removal from the body. 



78. Muscular Tissue. The remaining tissues are charac- 

 teristically active. Muscular tissue by its contractility has the 

 power of producing movements of the parts to which it is 

 attached. This contractility of muscle may be looked upon 



FIG. 27. 



FIG. 27. Plain muscle fibres, n, nucleus of muscle cell; p, undifferentiated cell proto- 

 plasm; p', the differentiated contractile portion of the cell. 



Questions on the figure. What are the two principal portions of these 

 cells? How do very young muscle cells compare with older ones in the 

 relative amount of these portions in the cell? Which is the more highly 

 differentiated portion? Where are such tissues found in the animal body? 

 Why are muscle fibres elongated? 



