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WORMS. 
The Thread-Worms, or Round-Worms, —Class Nematohelminth.es. 
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 
sriNY-HEADED thread-woem. 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 
