20 DEPARTMENT OF AGRICULTURE

a nematologist. Considering the comparative youthfulness of this branch
of science, it should be remembered that even the specialists' views1
concerning the relative significance of these forms are subject to aq
great deal of change and modification.
 It has already been pointed out that nematodes constitute an ex-1
tremely large and varied group of animals, divided into many different
families. With some exceptions ,the plant-parasitic forms belong to only
three of these families; the Tylenchidae, the Aphelenchidae, and the
Dorylaimidae. The members of all three of these families are provided|
with a stylet with which they feed. This organ appears to be fully effec-
tive in puncturing the plant tissues, and in obtaining the food required l
by these parasites by sucking. Furthermore, many types of plant-parasitic
nematodes appear to induce the host plant to produce their particular





























 F AB C E
 Figure 12.-Drawings of various developmental stages of a root-knot nematode.
A. Unsegmented egg; B. egg containing larva; C. migratory larva free in the soil;
D. sausage-shaped larva living sedentary in the root; E. larval molt containing fully
developed male; F. adult male; G. young female. X 140