324 ESSENTIALS OF ZOOLOGY 



quently make use of snails and crabs as hosts for a phase of their 

 life history. This group is divided into only three orders : Monogenea, 

 Digenea, and Aspidocotylea. CoUjlapsis, Paragonimus, Clonorchis, 

 Fasciola 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 

 muscular tissue of several different animals. The class includes four 

 orders : Bothriocephaloidea, Cyclophyllidea, Tetraphyllidea, Trypano- 

 rhyncha. Taenia, Diphyllohothrium, Hymenolepsis 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 newly 

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

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

 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 glands in the 

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

 larva is usually called pilidium. Prostonia, Cerehratulus, Tetrastema 

 are representatives. 



Habitat and Behavior of Planaria 



This free-living, fresh-water, flatworm thrives beneath the rocks, 

 logs, leaves, algae, or debris at the bottom of shallow spring-fed 

 brooks and pools. They must have pure, clear, cool water. These 

 animals are rather gregarious and when at rest will group together 

 beneath objects where the light is not intense. They respond nega- 

 tively to bright light. They usually feed upon minute plants and 



