198 THE INVERTEBRATA 



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 parasitic 

 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 con- 

 stitutes the lining of the gut there exist a large number of star-shaped 

 cells with large intercellular spaces forming a mass o{ parenchymatous 

 tissue. The nervous system consists essentially of a network as in the 

 Coelenterata, with the important diflference that there is an aggre- 

 gation of nerve cells at the anterior end which, in the free-living 

 forms almost always takes the form of 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 (Fig. 145). 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. 



By operating on the animals in different ways it is possible to show 

 what functions the different parts of the nervous system have. If the 

 cerebral ganglion of a Polyclad is removed, the body of the animal 



