JO Aug., 1910.] The Potato Eel-ivorm . 509^ 



one another, their rehitions to phmts and the fertility of soils are all points 

 requiring further research. 



To show the practical importance to the farmer and, indeed, to the 

 whole community of the Nematoda, it is only necessary to refer to one or 

 two cases. Crisp estimates that Syngamus trachealis (the cause of gapes) 

 is responsible for the death of half-a-million pullets in England every 

 year. Megnin states that in a single pheasantry 1,200 victims died daily. 

 The loss of one-third of the crop of sugar beet is by no means uncommon 

 when it is affected by Hcterodcra schachtii ; again it is no uncommon thing 

 in certain districts of Victoria to lose the bulk of the onion crop by the 

 ravages of Tylenchus devastatrix, and many bags of potatoes are con- 

 demned every year on account of their being affected by anotlier species of 

 the.se small worms. These cases show the practical importance of what, 

 at first sight, seem quite insignificant animals and the necessity for the 

 minutest observations ; for only when we are fully acquainted with the 

 life history of a parasite are we in a position to successfully combat it. 



Since time immemorial, crops of various kinds have died suddenly ; 

 so suddenly. Dr. Cobb remarks, "as to justify the expression ' struck by 

 lightning.' The unknown cause in some such cases has probably been the 

 gall worm. Many an agricultural or horticultural failure attributed to the 

 use of improper fertiliser, to poor soil, or wrong culrivation, has been due 

 to this insidious foe attacking the very fountain head of vegetation. Were 

 it possible in pounds, shillings, and pence, to sum up the damage done by 

 eel-worms, the total would probably amount to a fortune for a nation. 

 How little Ave know of the microscopic enemies and friends of vegetation 

 that ply their vocation in the vicinity of those important organs, the root 

 hairs."' 



Characters common to the Nematoda. 



Before going further it will be well to point out a few characteristics 

 common to the Nematoda. Von Linstow says that the only taw that can be 

 derived inductively from the study of the life history of nematodes is 

 " that those which live in animals never pass through all their stages of 

 development in the same organ." While this cannot be said of those para- 

 sitic on plants, still we often find something analogous ; for instance, 

 Tvlenchus tritici migrates from the stem and leaves up the plant to its 

 flower. 



We do not find the degenerations so commonly associated with para- 

 sitism in the nematodes affecting plants, there is no degeneration of the- 

 various organs necessary for a free life, nor is there any multiplication 

 and development of the organs of adhesion. The reproductive powers do 

 not seem to be increased ; thus we do not find the enormous numbers of 

 eggs commonly seen in some of the clas.ses of parasitic worms — they never 

 reproduce, so far as is yet known, a-sexually by budding, or fission, or 

 parthenogetically. The nematodes show less difference between the free 

 livmg and parasitic members of the group than obtains in any other class. 

 With few exceptions the parasitic forms have undergone little degeneration. 

 It is true they have no eyes such as some of the free forms possess, but 

 in other respects they do not show any marked retrogression ; further, the 

 mouth armature is developed in many of the free forms and is not confined 

 to the parasites. 



With one or two exceptions the nematoda are bi-sexual, and although 

 the males are as a rule smaller than the females, they show no trace of 

 degeneracy. 



