448 WORMS. 



The Thread- Worms, or Round- Worms, — Class Nematohelminthes. 



These worms are characterised by having a thread-like body, covered with 

 tough, elastic integument, but usually showing no distinct traces of being divided 

 into segments like those of leeches and earth-worms, and possessing no trace of 

 limbs. The sexes are generally distinct. The group is divided into the three 

 orders Acanthocephali, Nematoidea, and Chsetognatha. 



Spiny-Headed Thread- Worms, — Order Acanthocephali. 



In this order is contained the single genus Echinorhynchus, which is appro- 

 priately named for animals possessing a protrusible proboscis, armed with several 

 rows of backwardly-directed spines. The chief character in which this order 



spiny-headed thread-worm, a, Nat. size ; b, Head enlarged. 



differs from the next is the lack of any special alimentary canal and digestive 

 apparatus. In the adult stage the species of Echinorhynchus are found in the 

 intestines of vertebrates ; the large species figured above (E. gigas) infesting the 

 pig. But in order to reach its final residence in this host, it has to spend its early 



