DEPARTMENT OF AGRICULTURE


food requirements by injecting into the plant tissues the secretions of
their large esophageal or salivary glands. For this purpose the stylet
is used as an injecting needle.
 The stylet of the tylenchs and also that of the aphelenchs is called
a buccal stylet or "stomato-stylet." It is assumed to have originated
through the transformation of the buccal cavity of the ancestors of these
two closely related families. The sclerotized walls of the buccal cavity|
of these ancestors are assumed to have been amalgamated and so tol
have formed a perforated tube, or the buccal stylet presently seen in|
these forms. (Fig. 11, A & B). In the dorylaimids, however, which
represent a group of very different origin and relationship, the stylet
is assumed to be a transformed tooth and is therefore termed an "odonto-1
stylet." (Fig. 11 C). I
 The tylenchs and aphelenchs include the most outstanding plant
nematode pests, many species of which often occur in very large num-i
bers. The two families are differentiated mainly by one character. In
the tylenchs the dorsal esophageal or salivary gland empties into their
alimentary tract a short distance behind the buccal stylet (Fig. 11),'
while in the aphelenchs it empties into this tract in the middle esophageal q
bulb just in front of its valvulae.
 The dorylaims occur in numerous genera and species in soils every-j
where. Many live parasitically on and in plants, even in the leaves andl
other above-ground parts, but they rarely occur in such great numbers|
in an attacked plant as do certain -parasitic aphelenchs and tylenchs.q
All dorylaims appear to lead a migratory mode of life although some|
occur in plant tissues apparently quite sedentary, their body rolled upq
in a more or less tight spiral. I
 The most prominent characteristics of the tylenchs and aphelenchs,
 in comparing them with dorylaims, is decidedly greater size and more
 highly developed function of their oesophageal glands. These charac-
 teristics doubtless are directly related to the much more pronounced
 pathogenicity of the group. For instance, in many species of tylenchs'
 and aphelenchs, the secretions of these glands are known to have a|
 poisonous effect on the host in addition to inducing the host tissues to
 produce directly assimilable fluid for use as food by the parasite. Thea
 presence of these enlarged and strongly functional oesophageal glands
 in the tylenchs and aphelenchs appears, in general, to indicate an extra-i
 oral digestion and, correlated with it, a transformation of the intestines
 into an organ designed mainly for storage of food reserves. It is on
 this account that they are able to feed by inducing the host tissues,q
 through glandular injections, to produce directly assimilable food. The
 dorylaims, on the other hand,. suck the cell contents directly from thel
 host tissues; these cell contents, in turn, are digested and broken down
 to simple assimilable forms of food in their intestine.
 In reviewing the more important plant-parasitic nematodes, it appears4


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