SPONGES, FLAT WORMS, AND ROUND WORMS 153 



(1) The fertilized egg divides to form a group of cells. 



(2) These cells become separated into three classes: (a) flagel- 

 lated outer cells, (b) large non-flagellated cells either within or at 

 the posterior pole, and (c) undifferentiated cells between the other 

 two varieties. This embryo swims about with the aid of its 

 flagella. 



(3) The larva comes to rest, and the flagellated cells pass to 

 the interior, while the large non-flagellated cells migrate to the 

 outside. 



(4) The flagellated cells become the choanocytes of the adult; 

 the large non-flagellated cells become differentiated into the three 

 strata of the dermal layer; and the cells of the middle region 

 remain as the ameboid wandering cells and reproductive cells of 

 the full-grown sponge. 



2. FLAT WORMS PL AN ARIA 

 (Planar ia maculata Leidy) 



External Features. Planaria maculata (Fig. 76) is a flat 

 worm found only in fresh water, usually clinging to the underside 

 of logs or stones. Like most of the members of the Phylum 

 Platyhelminthes, its body is extremely flattened dorso-ventrally, 

 and is bilaterally symmetrical. Planaria is broad and blunt at 

 the anterior, and pointed at the posterior end. The length of an 

 adult specimen may reach half an inch. The body contains so 

 much coloring matter as to make the location of the internal 



4 



FIG. 76. Planaria polychroa. i, eye ; 2, side of head ; j, proboscis ; 

 pharynx sheath ; 5, genital pore. (From Shipley and MacBride.) 



