THE ORDER AMONG ANIMAL TYPES 107 



Protozoa by inexperienced observers. The whorls or circlets of 

 cilia at the anterior end are in constant motion sweeping food into 

 the mouth. The body wall is of three cell layers and is transparent. 

 The digestive system is a tube with both mouth and anal open- 

 ings. A specialized region of the tube, termed the stomach, contains 

 a pair of hard plates, that are in constant motion grinding the food. 

 This mechanism is readily observed through the body wall and its 

 presence unmistakably distinguishes a form as a rotifer. 



Phylum Molluscoidea 

 Whether or not the Bryozoa, or moss-like animals, and the 

 Brachiopoda, or mollusc-like animals, that form the two classes 

 of Molluscoidea should be thus joined is a matter of some dispute. 

 Some plumed Bryozoa (Fig. 43) that outwardly resemble Coelen- 

 terata are found in fresh water. The Brachiopoda (Fig. 44) live 

 within a bivalve shell similar in nature to the shells of Mollusca. 

 Their fossil remains show that these animals have existed for many 

 millions of years and were once much more numerous than now. 



Phylum Echinodermata 

 This phylum includes the Starfish and its allies, the most complex 

 animals in which radial symmetry characterizes the adult animal. 

 The structures are arranged in a radial pattern about a central 

 disk, the number of radii being usually five. The body is usually 

 covered with plates containing calcium carbonate, and is charac- 

 teristically spiny. The body is derived from three layers and the 

 mesoderm is separated into two sheets enclosing a body cavity or 

 ccELOM which is homologous with the body cavity of the higher ani- 

 mals. The alimentary canal has both mouth and anus. Locomo- 

 tion is by means of a mechanism peculiar to this phylum called the 

 AMBULACRAL WATER SYSTEM. It consists of a serics of canals containing 

 water, connected with tubes which end as feet with suckers oper- 

 ated as organs of attachment by means of bulbs on the walls of the 

 canals. Five classes are usually given: 



