CHAPTER IX 



THE MANY-CELLED ANIMAL BODY. DIF- 

 FERENTIATION OF THE CELL 



The many-celled animal body. In the study of cer- 

 tain of the tissues and organs of the toad we have learned 

 that the body of this animal is composed of many cells, 

 thousands and thousands of these microscopic structural 

 units being combined to form the whole toad. This 

 many-celled or multicellular condition of the body is true 

 of all the animals except the simplest, the unicellular 

 Protozoa. Corals, starfishes, worms, clams, crabs, in- 

 sects, fishes, frogs, reptiles, birds, and mammals, all the 

 various kinds of animals in which the body is composed 

 of organs and tissues, agree in the multicellular character 

 of the body, and may be grouped together and called the 

 many-celled animals in contrast to the one-celled animals. 

 This division is one which is recognized by many syste- 

 matic zoologists as being more truly primary or funda- 

 mental than the division of animals into Vertebrates and 

 Invertebrates. The one-celled animals are called Pro- 

 tozoa, and the many-celled animals Metazoa. 



Differentiation of the cell. It is apparent at first 

 glance that the cells which compose the body of a many- 

 celled animal are not like the simple primitive cell which 

 makes up the body of the Amoeba , nor are they like the 

 more complexly arranged cell of the Paramoecium. Nor 

 are they all like each other. The cells in the toad's blood 

 are of two kinds, the white blood-cells, which are very like 



