208 UNSEGMENTED "WORMS." 



and intestinal arteries of horse, causing aneurisms, colic, etc. The 

 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, 

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

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

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

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

 radish, cabbage. 



Class NEMATOMORPHA 



The Gordiidae (e.g. Gordius aquaticus the horse-hair worm) are so 

 different from true Nematodes that they must be ranked in a separate 

 class. There are no lateral lines. Three nerve-strands lie close 

 together in the mid-ventral line. In the adult Gordius the mouth is 

 shut and the food canal is partly degenerate. The adult Gordiidse 

 usually live freely in fresh water ; larval forms occur in aquatic 

 molluscs, young insects, etc. ; later stages usually occur in carnivorous 

 insects, whence they emerge to become adult in the water. One form, 

 Nectonema agile, is marine. 



ClaSS ACANTHOCEPHALA 



For a few genera, of which the best known is Echinorhynchus* 

 whose larvae live in Arthropods, and the adults in Vertebrates, a 

 special class, ACANTHOCEPHALA, has been established. They may be 

 placed beside Nematodes, but the relationship does not seem to be 

 very close. Mouth and gut are absent. The anterior end bears a 

 protrusible hooked proboscis used in boring in the intestinal wall of the 

 host. In the minute swellings at the ends of the two much-branched 

 excretory organs of E. gigas, there are ciliated cells, the only case 

 known among Nematohelminthes. 



Echiiiorhyw.hus proteus of pike, minnow, trout, etc., larva in the 



Amphipod Gammarus pulex. 

 ,, angustatus of perch, larva in the Isopod Asellus 



aquaticus. 

 moniliformis of rat, etc., larva in larval beetles 



(Blaps). 

 ff j&gas of pig, larva in grubs of cockchafer, etc. 



