266 



PRINCIPLES OF ANIMAL BIOLOGY 



Class I. Turbellaria. Free-living flatworms with ciliated epidermis. Planaria. 

 (Figs. 89, 221.) 



Subclass I. Rhabdocoelida 

 Subclass II. Tricladida 



Subclass III. Polycladida 



Class II. Trematoda. Parasitic flatworms without cilia but with a hardened 

 ectoderm, usually parasitic and with attaching suckers. Flukes. (Fig. 222.) 



Subclass I. Monogenea 



Subclass II. Digenea 



Class III. Cestoda. Parasitic flatworms with the body diff'erentiated into a 

 scolex, an enlargement usually provided with suckers and sometimes with hooks, and a 

 chain of similar structures (proglottides), the whole being usually regarded as a colony. 

 Tapeworms. (Fig. 223.) 



Phylum 5. Nemathelminthes. — These are elongated, bilaterally 

 symmetrical animals, commonly called round- or threadworms. They 



A B 



Fig. 224. — Important Nemathelminthes: A, Trichinella encysted in muscle; B, hook- 

 worm. (A from photograph by General Biological Supply House; B from Rivas, "Human 

 Parasitology," W. B. Saunders Company.) 



are triploblastic, and there is a "coelom" in the middle tissue layer. 

 The digestive tract, unlike that of the two preceding phyla, is not a 

 coelenteron, for it opens at both ends. There are no cilia on any part of 

 the body. The sexes are separate; that is, some individuals are males, 

 some females, none hermaphroditic. 



This is probably one of the richest phyla in numbers of species, but 

 its species are not proportionately well-known. Most of the members 

 of this group are free-living, and they are found in all sorts of situations, 

 in water or soil. Some infest plant tissues. Others are parasitic in 

 animals. The dread human disease called trichinosis is caused by round- 



