PRINCIPLES OF ANIMAL BIOLOGY 53 



4. Phylum PLATYHELMINTHES (Gr. platus = broad + helmins = 

 an intestinal worm). 



A. Characteristics 



4a. May be (1) free-living in water or earth, or (2) parasitic in or on 

 other animals. 



46. Bilaterally symmetrical. Examine Planaria. How many planes 

 may be passed through the center of the body, each dividing it into two 

 parts that are mirrored images of one another? 



4c. Triploblastic. Examine sections. Note a middle tissue between 

 ectoderm and entoderm. 



4d Single gastrovascular cavity (may be wanting in parasitic forms) . 

 Examine an entire planaria; note the gastrovascular cavity and its 

 branching form. It has but one opening, the mouth, as in the 

 Coelenterata. 



4e. Unsegmented. In Planaria, note that the body is not divided 

 into a series of segments. 



B. Special Features. 



In the tapeworm observe: 



4/. That the animal is not segmented, but is colonial, the members of 

 the colony being attached in a linear series. Each individual is called 

 a proglottis. 



4gr. The head or scolex. The individuals of the colony are successively 

 budded off from the scolex. Note the hooks: also the suckers. What is 

 the use of these structures? 



4/i. The absence of a gastrovascular cavity. Why is it not necessary? 



4i. The reproductive bodies make up the greater part of the body. 



5. Phylum NEMATHELMINTHES (Gr. nema = thread + helmins = 

 an intestinal worm) . One of the richest phyla in number of species, yet 

 seldom attracting general attention. 



A. Characteristics. 



5a. Cylindrical in form. See Ascaris or any other species available. 



56. Bilaterally symmetrical. Meaning of this expression? Verify 

 in Ascaris or any other species. 



5c. Triploblastic. Meaning of this term? Transparent living forms 

 can usually be had either in mother of vinegar (vinegar "eels") or in 

 old protozoan cultures. If the nematodes keep quiet enough, observe 

 the triploblastic feature. Or examine a cross-section. 



5d. Unsegmented. See any nematode, e.g., Ascaris. 



5e. Alimentary canal with both mouth and anus. Demonstrations 

 may be available. 



5/. Coelom present. This is a cavity surrounding the alimentary 

 canal. Examine a dissection of Ascaris, and observe that the body wall 



