MULTICELLULAR ANIMALS 135 



ever> are almost all radially symmetrical. The most complex animals 

 that exhibit this character are the starfishes and their allies, the 

 members of the phylum Echinodermata. Most are familiar with the 

 general shape of the common starfish (Fig. 45). While it is truly 

 radially symmetrical, this animal finds it necessary in its locomotion 

 to have a permanent orientation, so that when the animal moves a 

 certain ray always leads. 



The more active and advanced animals are built upon some modi- 

 fication of bilateral symmetry, oriented in such fashion that the 

 longest dimension coincides with the line of usual movement. It has 

 been found necessary to describe the location of various parts and 

 regions of a bilaterally symmetrical animal by a set of terms. Thus, 

 the leading end, or controlling region is called the anterior end, 

 the opposite end being the posterior (Fig. 80). That region of the 

 body which is usually presented upward is called the dorsal region, 

 or dorsal surface, as the case may be. The nether surface is spoken 

 of as the ventral surface. If one imagines a plane passed through a 

 bilaterally symmetrical animal dividing it into right and left halves 

 by passing through the anterior and posterior ends and at the same 

 time coinciding with the mid-lines of the dorsal and ventral sur- 

 faces, then each half is a mirror image of the other (Fig. 80). Parts 

 that fall outside this plane of symmetry are spoken of as lateral with 

 reference to the plane. All descriptions of the location of parts 

 employ the adjectives anterior and posterior, lateral and medial, 

 dorsal and ventral. The adverbs or adjectives cephalad and ce- 

 phalic are also used to refer to anterior structures; caudad and 

 CAUDAL are also used to refer to posterior positions. 



The Body Cavity. In most forms more complex than the 

 diploblastic animals the mesoderm, the intermediate layer that de- 

 velops between the ectoderm and endoderm, in an early embryonic 

 stage splits into two sheets. The sheets are separated by a cavity and 

 the outer sheet becomes closely applied to the inner surface of the 

 ectoderm, while the inner sheet becomes associated as the outer 



