134 



ELEMENTS OF BIOLOGY 



that are directly concerned with the chemical modification of the 

 foods, a function that is solely endodermal from the most simple 

 type of multicellular animal to the human body. The muscular ele- 

 ments of the digestive systems of all animals, like all muscles, are 

 derived from mesoderm. 



Symmetry. The forms assumed by Metazoa are grouped into 

 only two main types of symmetry, radial and bilateral. A form is 



LATERAL 

 ANTERIOR >. II POSTERIOR 



MEDIAL 



Fig. 8o. — Diagrams to show the planes of symmetry and orientation in radial and 



bilateral forms. 



spherically symmetrical if its parts are identically placed with refer- 

 ence to a point. Among the Protozoa a colony of volvox is spheri- 

 cally symmetrical, as are also typical metazoon blastul^e (Fig. 77), 

 but no spherically symmetrical adult Metazoa occur. In the diplo- 

 blastic animals, Porifera and Ccelenterata, the symmetry is radial, 

 that is, the parts are symmetrically arranged in imaginary planes 

 which pass through a common axis (Fig. 80). Such organisms 

 appear to be ineffectively oriented for locomotion and this type of 

 symmetry is not extensively characteristic of animals. Plants, how- 



