CHAPTEE VIII 



PHYLUM VI. : PLATYHELMINTHES OR 

 FLATWORMS 



FLATWORMS are small, rather worm-like creatures, with a soft, 

 flattened, bilaterally symmetrical body, either unsegmented, 

 or with segments having very little connection with each other. 



In members of this group we find three body-layers well 

 developed, but, unlike true worms, they have no body-cavity 

 or coelom outside the alimentary canal or gut ; the mesoderm 

 forms a kind of connective tissue entirely filling the space 

 between the gut and the skin. 



Microscopic examination of stained preparations of the 

 body, reveals a fairly complex structure, with a specially 

 peculiar excretory vascular system. 1 



Turbellaria The best-known Flatworms are the Turbellaria 

 or Whirl- or " Whirl-worms," little forms never exceeding an 

 worms. j ncn j n length, and often much smaller than this, 

 which crawl over the ground like slugs, or swim by the 

 whirling motion of the cilia which cover their bodies. Such 

 whirl-worms are found both in sea-pools and in ponds, where 

 they are useful as scavengers, for they feed on dead animal 

 matter. They have a much-branched digestive system, with 

 three main branches to the gut in the fresh-water forms, but 

 more in many marine species. The alimentary canal is 

 peculiar in having no anus. 



The commonest marine form is Leptoplana tremellaris (Fig. 

 48), which is to be found under stones or seaweed when the 

 tide is out. It has a flat, semi-transparent body, J to 1 inch 

 in length, which can either glide quickly over a surface, or swim 

 freely in the water with a wave-like up-and-down motion. 



1 See Cambridge Natural History, vol. ii. pp. 25 and 41. 

 94 



