ACANTHOCEPHALA. 185 



by another, pig eating rat, man eating pig. In the alimentary canal of 

 the new host the capsule is dissolved, the embryos are set free, and be- 

 come rapidly reproductive. 



Among the numerous other parasitic Nematodes the following may be 

 noted : — The giant palisade v/oxxn (Enstimigy /us gigas) occurs in the renal 

 region of domestic animals, &c. ; the female may be 3 feet long. The 

 armed palisade worm (Strongylus annatiis) occurs in the intestine and 

 intestinal arteries of horse, causing aneurisms, colic, &c. 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, &c. The large Ascaris 

 mcgalocephala and the much smaller Oxyuris cwzntia are not uncommon 

 in horses. Syngamus trachealis occurs in the trachea of birds, causing 

 "gapes." Various species of Tylenchus, especially T. devastatrix and 

 T. staitdens (or T. tritici), destroy cereal and other crops. Various 

 species of Hcterodera (especially H. schachtii ax\A H. radicicola) infest the 

 roots of many cultivated plants — e.g., turnip, radish, tabbage. 



Classification. 



At present the Nematodes are usually classified in families — Ascarida;, 

 AnguillulidL"e, &c. With these we need not concern ourselves here, but 

 it is important to notice that the Gordiida;, {e.g., Go7-dius aqiiaticiis — the 

 horse hair worm) are very different froin all the others. In the adult the 

 mouth is shut and the food canal is partly degenerate. The adults live 

 freely in fresh water ; there are two larval forms, the first in aquatic 

 molluscs, young insects, &c. , the second in adult insects, fish, frog, &c. 



ClaS.S AcANTHOCEPHALA. 



For a single genus Echinorhynchits, whose larvje. live in Arthropods, 

 and the adults in Vertebrates, a special class, Acanthocephala, has 

 been established. We may provisionally place this genus, which has 

 about a hundred species, beside Nematodes, but the relationship does 

 not seem to be very close. Mouth and gut are absent. The anterior 

 end Ijears a protrusible hooked proboscis. 



Echinorhynchtis proteiis of Pike, larva in the Amphipod Gaiiiiiiarus pukx. 

 ,, angiistatus of Perch, larva in the Isopod Asellus aqiia- 



ticiis. 

 ,, i^^s'^-^ f>f P'gi larva in young Cockchafers. 



