The Animal Kingdom 345 



Symmetry. — The arrangement of the parts of an organism in 

 relationship to one another is known as symmetry. In some forms it 

 is impossible to divide the organism into equal parts by planes ; such 

 organisms may be said to be asymmetrical. Those animals having 

 the body in the form of a sphere, with the parts radiating from a central 

 point, have spherical symmetry. When the body is cylindrical in shape, 

 with the parts arranged along a longitudinal axis, the animal is said 

 to be radially symmetrical. An indefinite number of planes can di- 

 vide these animals into equal halves. Biradial symmetry is a modifica- 

 tion of radial symmetry. In it, the mouth area is elongated and there 

 are but two possible planes of symmetry. The type of symmetry typical 

 of the largest number of animals is bilateral; in this, the two sides of 

 the body are mirror images of one another. Only one plane can divide 

 the body into equal and identical halves. 



ASYMMETRKAL 



Fig. 110. — Types of symmetry. 



Germ Layers. — The protozoans, being of a cellular grade of con- 

 struction, have no germ layers. All of the metazoans, except the 

 Mesozoa, possess embryonic layers of tissues : the ectoderm, mesoderm, 

 and entoderm. These are variously modified in the adults. At the 

 lowest level, the mesoderm consists only of mesenchymal cells de- 

 rived from the ectoderm. It is not differentiated into organs, and in 

 some instances may even be lacking. Among those phyla considered 

 to be higher in their development than the coelenterates and the cteno- 

 phores, the mesoderm is derived primarily from entodermal material 

 and is differentiated into organs in the adults. 



Presence of a Body Cavity. — During the early stages of em- 

 bryology, the I)lastula has a large central cavity, the blastocoel. After 

 gastrulation, the entoderm forms the primitive digestive tract in this 



