SCOLEOIDA. 



109 



4. Acanthocephala (Thorn-headed worms). 



5. Gordiacea (Hair-worms). 



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



7. Hotifera (Wheel-animalcules). 



Oedek I. T^NiADA (Gr. tainia, a ribbon). — In this order 

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



Fig. 39. — ^Morphology of Tseniada. 1. Ovum containing the embryo in its leathery case; 

 2. A bladder-worm {OyaUcercus longicollds), magnified ; 8. Head of the adult T(Enia 

 eoUum, enlarged, showing the suckers and crown of hoolclets; 4. A single generative 

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

 vascular canals (&) ; 5. Fragment of Taenia Bolium, 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. 



