60 Invertebrata. 



in the smaller flatter forms is often tree-like, branched 

 (fig. 35). In others it is a simple pouch with no 

 excretory orifice, but in the larger forms it is elon- 

 gated. The water-vessels appear as two lateral tubes, 

 and the egg-producing organs are usually complex. 

 The young Turbellarians on leaving the egg are usually 

 unlike the parent, often helmet-shaped, with a whip- 

 lash-like process at the apex, but this larva develops 

 into a worm-like body by moulting or shedding its 

 surface. The smaller forms are generally flattened, 

 somewhat elliptical ; the largest are worm-like, some- 

 times very long, and are called Nemertean worms. 

 One of these, Borlasia, found not very uncommonly 

 on our own shores, has been taken measuring twelve 

 feet in length. 



The 2nd, 3rd, and 4th classes of worms are mostly 

 parasitic in their habit and are called suctorial, round, 

 and thorn-headed worms respectively. 



CLASS II. Cotylidea. The sucker-bearing worms 

 are so called because they are armed with rounded or 

 irregular cup-like suckers. These worms are generally 

 simple in organisation ; and their body cavities and 

 digestive organs are either abortive or rudimentary. 

 The two types of these worms are Tapeworms and 

 Flukes. 



The tapeworms are so called from their great 

 length and flatness. They exist principally in the 

 digestive canals of higher animals, especially in fishes. 

 The human race is not exempt from occasionally 

 harbouring at least three species of these parasites. 

 One of these, the common tapeworm, or Tania solium, 

 is common in Britain and Western Europe, and may 



