UNSEGMENTED WORMS 205 



The Turbellarians are free living animals and may be terrestrial, 

 fresh-water or marine; the Rotifers are as a rule free-swimming 

 and occur chiefly in fresh water; the Polyzoa are aquatic, 

 attached, colonial forms but lead for the most part an independ- 

 ent existence, or may occasionally be commensal with other 

 types of animals; the Brachiopods are marine and may be 

 attached, but are not colonial; the Trematodes and Cestodes 

 represent all kinds and degrees of parasitism. Even if all 

 these classes of animals could be considered akin, their habits 

 of life and their consequent adaptations are so various as to 

 produce the greatest range of general form and special structure. 

 If we consider the relatively small number of species of 

 animals in these groups, the species of the Platyhelminthes are 

 among the most widely distributed of the Metazoa. This is 

 true both of the free Turbellaria and Nematoda and the para- 

 sitic Trematoda and Cestoda. There is probably not a large 

 species of the higher Metazoa which escapes being the host of 

 one or more of these worms at some stage of its life history. 

 The fact of parasitism, the ability to carry on the life cycle in a 

 series of hosts, and the prevalence of the carnivorous habit 

 among its hosts all help the distribution. The organs more 

 commonly infested by the parasites are the digestive tube, the 

 blood and lymphatic vessels, the coelomic cavity or other 

 organs where the nutritive fluids of the body are abundant. 

 They produce all sorts of disorders from mere functional dis- 

 turbance (such as digestive disorders and anaemia from the 

 presence of the tape-worm) to the destruction of the tissues of 

 the organs involved. It is very commonly true that the adult 

 or sexually mature individuals are produced in one host, and 

 the eggs or larvae produced by them find their way into another 

 species of host where a portion of the development toward 

 maturity occurs. The transfer of the parasite from the second 

 back to the first host-species is necessary to complete the cycle. 

 In size the unsegmented worms vary from minute microscopic 

 dimensions to thirty feet in length in the tape-worm, Diphyllo- 

 bothrium latum. Some suggestion of their importance to man 

 and the higher animals may be gathered by reference to the 

 following table (p. 206). 



