THE THREE-LAYERED ANIMALS 



161 



one that was well on its way toward higher 

 forms. 



Of the three classes of the phylum Plat\'- 

 helminthes only the class Turbellaria in- 

 cludes free-living animals. Members of this 

 group may be found among the rocks in 

 cool streams or ponds, or upon the shady 

 side of submerged plants. Turbellarians are 

 carnivorous, feeding on small animals, 

 either living or dead. There are also marine 

 forms in tliis group which sometimes live 

 in the intestines of sea urchins and other 

 forms of ocean life. 



The other two classes of the phylum are 

 the Trematoda, or flukes, and the Cestoidea, 

 or tapeworms. All members of these two 

 classes are parasitic and will be described 

 further in a later section, but first let us 

 examine the free-living turbellarians. 



PLANARIA 



The study of tripoblastic, or three-lay- 

 ered, animals may well begin with planaria, 

 a common inhabitant of North American 

 streams. It seeks shelter in darkened, se- 

 cluded spots and comes out at night to 

 move around in the cool waters in search 

 of food. Planaria is flattened dorsal-ven- 

 trally and is darkly pigmented. It is cov- 

 ered with cilia, which enable it to glide 

 along the substratum over a mucous path 

 (Fig. 9-1) secreted by glands on the ventral 

 surface of the body. By use of a muscle 

 layer developed in the mesoderm, planaria 

 can crawl in true worm-like fashion. Cer- 

 tain muscle groups produce a twisting mo- 

 tion so that it sometimes appears to raise 

 its head and look about. Although it may 

 appear that planaria can see when this oc- 

 curs, actually its two large eye-spots form 

 no image and are only sensitive to varying 

 intensities of light. 



Unlike most heads, that of planaria has 

 no mouth, for the mouth is located on the 

 ventral side of the body near the middle. 

 It opens into a muscular pharynx, which 

 lies in a sheath extending anteriorly, and 



mucous 

 path 



Fig. 9-1. Planaria crawling and feeding. 



when planaria is hungry and in search of 

 food, it often protrudes its pharynx and 

 moves about with it thus extended. When a 

 small piece of meat is tossed into the water, 

 hungry planarians attach themselves to it. 

 A digestive fluid pours out from the phar- 

 ynx to aid in the disintegration of the meat. 

 The partially digested food is then taken 

 into the digestive tract where digestion is 

 completed. Planaria's chief food consists of 

 small Crustacea. In this case the epidermal 

 slime glands secrete a sticky substance 

 which is sprayed over the victim, rendering 

 it helpless. It is usual for planaria to grip 

 its prey with the head region first and then 

 attach its muscular pharynx to the food, bits 



