THREAD- WORMS 



241 



young forms are swallowed from stagnant water, bore from gut into 

 arteries, become adult, return to gut, copulate and multiply. Various 

 other species of Strongylus occur in sheep, cattle, etc. Of the genus 

 Ascaris alone, over 200 species have been found in all types of Verte- 

 brates : — A. megalocephala in horses, A. lumbricoides in man, A. mystax 

 in cats and dogs. Syngamus trachealis occurs in the trachea of birds, 

 gapes," e.g. in poultry and pheasants. It pierces the wall 



causmg 



Fig. 131 . — Trichinae in miiscle, 

 about to be encapsuled. — 

 After Leuckart. 



Fig. 132. — Trichinae in muscle, 

 encapsuled. There may be 

 12,000 in a gramme of pig's 

 muscle. — After Teuckart. 



of the trachea, and " actually clenches the teeth with which its mouth 

 is provided in the tracheal rings." A remarkable large form, Ichthyo- 

 nema grayi, is found inside sea-urchins. Various species of Tylenchus, 

 especially T. devastatrix and T. scandens (or T. tritici), destroy cereal 

 and other crops. Various species of Heierodera (especially H. schachtii 

 and H. radicicola) infest the roots of many cultivated plants, e.g. turnip, 

 radish, cabbage. 



There is evidently a great variety of habit and habitat 

 among Nematodes, and yet the general structure is very 

 uniform. They seem to represent a homogeneous class, 

 very much by themselves, and not nearly related to other 

 types, even to the other Nematohelminthes. 



16 



