CHAPTER VI 



THE PHYLUM PLATYHELMINTHES 



Free-living, bilaterally symmetrical, triploblastic Metazoa; usually 

 flattened dorsoventrally ; without anus, coelom or haemocoele; with 

 a flame-cell system; and with complicated, usually hermaphrodite, 

 organs of reproduction. 



The name Platyhelminthes is given to a division of that hetero- 

 geneous collection of animals, which in Linnaeus' time were called 

 Vermes. The Vermes included everything that looked like a worm, 

 but appearances have since been found to be deceptive and the 

 collection has been broken up into separate phyla, one of which is the 

 Platyhelminthes or flatworms. Of all the worm-like animals the flat- 

 worms are undoubtedly the most primitive, for they alone show 

 relationships to the Coelenterata. 



The phylum Platyhelminthes falls naturally into three classes: 

 (i) Turbellaria, (ii) Trematoda, (iii) Cestoda. 



Of these the Turbellaria are with few exceptions free-living, while 

 the Trematoda and Cestoda are all, without exception, parasites. It 

 is in the Turbellaria that we see most clearly the typical organization 

 of a platyhelminth, for in the Trematoda and Cestoda the parasitic 

 habit has induced a considerable departure from the structure of the 

 free-living ancestor. In shape the Platyhelminthes are flattened, they 

 are not segmented and do not possess a coelom. The ectoderm is 

 ciliated in the Turbellaria, but the ciliation is lost in the two para- 

 sitic groups and there are further modifications. The gut, which is 

 present only in the Turbellaria and Trematoda, has but one opening 

 which serves both as mouth and anus, and in this respect reminds us 

 of the Coelenterata. Between the ectoderm and the endoderm which 

 constitutes the lining of the gut there exist a large number of star- 

 shaped cells with large intercellular spaces forming a mass oi paren- 

 chymatous tissue. The nervous system (Fig. 141) consists essentially 

 of a network as in the Coelenterata, with the important difference 

 that there are always a pair of cerebral ganglia and that certain of 

 the strands of the network stretching backwards from these cerebral 

 ganglia are often more distinct than others and merit the name of 

 nerve cords. There is, therefore, the beginning of a definite central 

 nervous system. There are no ganglia other than the cerebral, but in 

 the general nervous network nerve cells and nerve fibres are mixed 

 together. 



Sense organs occur in adults only in the free-living Turbellaria, 



