PHYLUM PLATYHELMINTHES 167 



may also be other suckers on other parts of the body. The trematodes 

 are all parasites. 



3. Cestoda (ses to' da; G., kestos, girdle, and eidos, form). — The 

 members of this class are also provided with a thick cuticular covering. 

 They have a so-called head, or scolex, provided with suckers and in 

 many cases also with hooks. The body is divided into a series of sections 

 or proglottids which vary greatly in number in the different species. As 

 a result of parasitism, which also prevails in this group, many organs 

 are reduced and the digestive system is entirely absent. 



194. Turbellaria. — In addition to the fresh-water planarians this 

 class includes a great many free-living marine flatworms. These may 

 be found making their way over the surface of rocks or other solid objects 

 in the water and at low tide are often found closely adherent to the lower 

 surfaces of rocks lying upon the beach. In the latter case they are 

 difficult to detect because they make a very thin film and the mottling 

 of the body is quite similar to the mottling of the surface to which they 

 adhere. Some of these marine forms are relatively large, reaching a 

 length of several inches, and are broadly oval or elliptical in outline. 

 On the other hand, there are some which are so small as to be microscopic. 

 The enteron of the more minute forms is simple and unbranched, while 

 that of the larger forms is divided into one or more main branches. These 

 in turn give rise to very complexly divided lesser branches which reach 

 all parts of the body. Turbellarians are not confined to water, either 

 salt or fresh, for in the tropics there are species which live in and on moist 

 earth. The fresh-water forms may be in quiet water or in swiftly flow- 

 ing streams; they may be collected under the ice in winter and have also 

 been found in hot springs at a temperature of 47°C. (116.6°F.). 



The larger forms are of various shades of gray, brown, or black, 

 but the smaller ones are often brightly colored and may be green from 

 the presen(!e of symbiotic algal cells in the parenchyma. The eyespots 

 may be absent, but the usual number is two; and one form, Polycelis. 

 has a large number. Olfactory pits and statocysts may also be present. 

 The turbellarians are richly supplied with glands. Some of these secrete 

 a slimy mucus; others, at the ends of the body, act as adhesive cells; 

 and still others produce material which forms rhabdites or rodlike crystal- 

 line bodies which are thought to serve as a means of defense or of cap- 

 turing food. 



195. Trematoda. — This class includes animals generally known as 

 flukes. They are parasitic on and in a great variety of other animals, 

 as on the skin of the salamander and certain fishes, on the gills of fishes 

 and tadpoles, and in several internal organs of many vertebrates. The 

 number of suckers and their location differ in different types. The 

 pharynx is not protrusible but it is muscular and capable of suction. 

 The type chosen to illustrate this class is Clonorchis sinensis Cobbold 



