40 BRANCH PLATYHELMINTHES 



an alimentary tract with an anal opening and a distinct blood- 

 vascular system.' They are usually dioecious.^ 



Geographic Distribution. — This branch of animals is the most 

 widely distributed of any above the protozoans. They are 

 found on land, in streams, and in the depths of lake and sea. 

 The parasitic forms are found in some stage in almost every class 

 of metazoans, while others have a commensal life with ascidians. 

 All are carnivorous. 



Economic Importance. — Many domestic animals are hosts 

 for these parasites and much loss is occasioned thereby. A 

 number of class Cestoda are parasitic in man and cause annoy- 

 ing if not dangerous diseases. The only sure preventive of these 

 parasites is to have all meats thoroughly cooked and fruits and 

 vegetables well washed. 



Important Biologic Facts. — An anterior end — one placed 

 foremost in locomotion — and a posterior end appear for the first 

 time in platyhelminthes. Also right and left and dorsal and 

 ventral sides are found. 



In the Nemertinea there is an alimentary tract with a mouth 

 and an anal opening. There is no distinct coelom. 



Class Turbellaria is the most primitive and the most closely 

 related to the Coelenterates, but it is not thought to be derived 

 from them, though it shows special points of resemblance to the 

 Ctenophora. It is thought that Trematoda and Cestoda are 

 descendants of Turbellaria. In Trematoda is seen an alterna- 

 tion of generations consisting of the succession of several dis- 

 tinct generations in regular series. Such an alternation of gen- 

 erations is termed heterogeny. The simple structure of parasitic 

 forms illustrates the principle that easy life — one requiring 

 little exertion — is accompanied by a low stage of development. 



Classification. — 



Class. Examples. 



I. Turbella'ria. Planarians. 



II. Tremato'da. Liver-fluke. 



III. Cesto'da. Tapeworms. 



IV. Nemertin'ea. Carinella, Tetrastemma, etc. 



1 MoMurrich, p. 160; Osborn's "Economic Zoology," p. 85; Kingsley's 

 Hertwig, p. 289. 



2 " Invertebrate Zoology," McMurrich, p. 162 ; Parker and Haawell, p. 279. 



