PHYLUM PLATYHELMINTHES 161 



Class Trematoda (tremato'da — having pores). — These animals, 

 commonly called flukes, have no epidermis but a thick nonciliated 

 cuticle. The body is either leaf-shaped or elongate and has from 

 one to many ventral suckers. This entire class is parasitic, and the 

 immature stages frequently make use of snails and crabs as hosts 

 for a phase of their life history. This group is divided into only 

 two subclasses : Monogenea and Digenea with orders Gasterostomata 

 and Prosostomata. Paragonimus, Clonorchis (Fig. 397), Fasciola 

 (Figs. 398 and 399) are genera representing the class. 



Class Cestoda (ses to'da — girdle form). — This group is also char- 

 acterized by a heavy cuticular cover, and a long, ribbonlike body 

 divided into sections called proglottides. 



These tapeworms each have a knoblike "head" or scolex on the 

 anterior proglottid. This structure is supplied with suckers for at- 

 tachment and sometimes has hooks. There is no alimentary tract, 

 and the group is parasitic. A developmental stage of the life history 

 is the bladder worm or cysticercus which lives embedded in the mus- 

 cular tissue of several different animals. The class includes five 

 orders: Pseudophyllidea, Cyclophyllidea, Tetraphyllidea, Trypano- 

 rhyncha, and Heterophyllidea. Taenia (Figs. 400, 401, and 402), 

 Diphyllohothrium, Ilymenolepis are examples. 



Class Nemertina. — It seems difficult to know where to classify 

 this group since some systematists give it the rank of phylum while 

 others give it lower ranking. The Nemertinea (nem er tin'e a— 

 unerring) as individuals, are unsegmented "band worms." Most 

 of them are free living and marine. A long proloscis, the neioly 

 developed blood vascular system, the alimentary canal, two apertures, 

 and cilia over the body are all characteristic of this type. There is 

 present a mesoderm, nervous system, and excretory system, but there 

 seems to be no coelom. The animals feed on the bodies of other 

 animals and on certain types of general organic matter. They 

 usually live in burrows in sand or mud or beneath solid objects. 

 The larger ones reach a length of ninety feet. The animals are fre- 

 quently brightly colored. There are numerous mucus-secreting glands 

 in the skin which may produce a tubelike dwelling for the worm. The 

 two muscular layers of the body are so effective that an extended 

 worm of fifteen feet may contract to less than two feet in length. 

 Locomotion is accomplished by the cilia and the contractility of 

 the body. The proboscis is a very characteristic organ which is 



