SCOLECIDA. 109 



4. Acanthocephala (Thorn-headed worms). 



5. G-ordiacea (Hair-worms). 



6. Nematoda (Round-worms and Thread- worms). 



7. Rotifer a (Wheel-animalcules). 



ORDER I. T^EOTADA (Gr. tainia, a ribbon). In this order 

 are comprised the ribbon-shaped Tape-worms (Fig. 39, 5) and 



FIG. 39. Morphology of Taeniada. 1. Ovum containing the embryo in its leathery case; 

 2. A bladder-worm (Cysticercus longicollis), magnified ; 3. Head of the adult Tomia 

 soliwm, enlarged, showing the suckers and crown of booklets ; 4. A single generative 

 joint, enlarged to show the branched ovary (o), the generative pore (a), and the water- 

 vascular canals (6) ; 5. Fragment of Toenia solium, showing the generative joints and 

 the alternate arrangement of the generative pores. 



the bladder-worms or cystic worms (Fig. 39, 2). These were 

 formerly described as distinct groups ; but it is now known 

 that the latter are merely the young forms of the former. The 

 peculiarity which distinguishes the development of the Tceni- 

 ada, and which led to the cystic worms being described as 

 distinct animals, is that the different stages of growth are 

 always found inhabiting different animals or " hosts." If the 

 fully-grown tape- worm is found in one animal, then its young 

 form or cystic worm will always be found in another. Many 

 animals are infested by tape-worms ; but all the leading points 

 of interest in the order will be brought out by a consideration 

 of the commonest of the three tape-worms to which man is 

 subject namely, the common tape-worm, or Tcenia solium. 



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