NEMATODA 253 



that of the species of Oxyuris in which the tgg is swallowed by the 

 host and the remaining stages of development take place in the gut. 

 It is said that several successive generations of the parasite may occur 

 within the same host. On the other hand, the wandering habit of 

 nematodes is a fundamental character and even forms in the first stage 

 of parasitism (facultative) may penetrate host tissues. 



The life histories of the principal nematode parasites of man and 

 domestic animals are summarized on pp. 254-5. They are arranged 

 in a definite order passing from the simplest type in Haemonchiis to 

 the most specialized life histories in Filaria. 



Two other classes of nematode parasites merit particular attention. 

 They are, respectively, parasites of plants and insects. 



Plant parasites. Nematodes are particularly fitted for a parasitic 

 life in plants by reason of their form and activity and their capacity 

 (at the end of the second larval stage) for resisting desiccation and 

 other unfavourable conditions. They are small enough, as larvae, to 

 obtain entrance through the stomata of leaves, and sometimes possess 

 dart-like projections of the buccal lining which enable them to 

 penetrate the cell walls of plants. They feed on cell sap and by 

 their interference with the life of the host plant cause the for- 

 mation of galls, wilting and withering of the leaves, and stunting of 

 the plant. 



Tylenchus tritici passes through a single generation in the course of 

 the year, and infects wheat. The animal becomes adult when the grain 

 is ripening and a pair, inhabiting a single flower, produce several 

 hundred larvae. Instead of the grain a brown gall is produced, and 

 in this the larvae (after moulting twice) may survive for at least 

 twenty years. If the grain falls to the ground the larvae may remain 

 there over the winter or may escape into the soil. When the corn 

 begins to grow in the spring they enter the tissues of the plant and 

 make their way up the stem to the flower, where they speedily mature. 

 The great interest of this life history lies in the easy adaptation of the 

 parasitic life history to the annual cycle of the wheat plant and the 

 extreme capacity for survival in a dormant and desiccated condition 

 until the right plant host becomes available. Tylenchus devastatrix, 

 on the other hand, may pass through several generations in the year 

 and attacks indiscriminately clover, narcissi bulbs and onions, and 

 many other useful plants. Heterodera (Fig. 178 D) is a parasite of the 

 roots of tomatoes, cucumbers and beets, and is remarkable because 

 the female attaches herself in larval life to a rootlet from which she 

 sucks a continuous flow of sap. She is fertilized by wandering males 

 and grows enormously, becoming lemon-shaped. Inside the body 

 thousands of larvae are produced, which escape into the soil and live 

 there until the opportunity arises for infection of fresh roots. 



