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DIGESTIVE SYSTEM

The digestive tract consists of the following principal regions: stoma or buccal cavity, oesophagus, intestine, rectum and anus. The oral opening of the stoma is surrounded by lips, usually six in number, designated as subdorsal, lateral and subveotral. In plant parasitic nematodes, the number of lips as well as the sensory structures may be reduced in numbers. The stoma may be very simple or quite elaborate—a simple plain walled tube with a spear or complicated cavity with teeth or denticles, etc., for puncturing, masticating or tearing. The anterior part of the digestive tract, comprising of buccal cavity (stoma), oesophagus and the oesophagus-intestine valve, is also referred as the foregut or stomodeum or stomatodaeum. The mid-gut or the intestine is the mesentron, the faindgut or the proctodaeum comprises of the intestine-rectal valve, the rectum in the females and cloaca and its associated structures in the case of males. The oesophagus and the rectum are differentiated from the intestine by their cuticular lining. The digestive tract as such, from the oral to anal openings, appears as a simple tube. The characters and the morphology of the stoma as well as the oesopha­gus are of taxonomic value and have a primary importance in all systems 01" classification.

    All plant parasitic nematodes possess a stylet. In the Secernentea (Pbasmidia), this is considered to be formed by the coming together of the sclerotisation of the buccal capsule and accordingly termed as stomatostylet or stornatostyle. In Adenophorea (Aphas-midia), the anterior part of the stylet is representing an enlarged tooth that originates in the oesophagus wall. This is termed as odontostylet or odontostyle. The posterior part of the stylet is derived from the lining of the oesophagus and is termed as odontostylet extension. After initiation of the odontostylet from the oesophagus wall, it moves forward to its position in the stoma with each successive moult.

    Typically, the stylet of a plant parasitic nematode is composed of three main parts—an anterior conical part, the middle shaft and the three knobs, one dorsal and two sub-ventral. The protractor muscles for the stylet are attached to the posterior part of the stylet. At each moult, during the larval stages, the anterior part of the stylet is shed off along with the cuticle and the remaining two parts disappear. The new stylet is formed with the emergence of the conical part first followed by the latter two parts. In the plant parasitic nematodes, stylet is an essential part of the feeding process by the nematodes. The protractor muscles, attached to the stylet, help the stylet in movement, in and out of the stoma. In some of the plant parasitic nematodes, stylet is weak with no basal knobs (genus Aphelenchus) or with weak knobs fas in genus Aphelenchoides) or with strong massive knobs (as in genus Hoplolaimus). The males of some groups have a degenerated stylet or do not possess any stylet but the juvenile forms always exhibit a stylet. In the tylenchs, the stylet is a hollow, tripartite and articular structure while in the dorylaims the stylet is generally rot hollow.

Oesophagus. Next to stoma is the oesophagus which varies in Structure in different groups of nematodes. It is, therefore, of great

taxonomic value. It is a tube lined by thin cuticle and covered by a membrane. The lumen of the oesophagus is triradiate and is extended into three symmetrically arranged longitudinal grooves that partially divide the oesophagus wall into three sections, one dorsal and two ventro-laterals (sub-lateral). However, the triradiate oesophageal lumen may not be present throughout the tube in all the parasitic nematodes. It is cylindrical in the anterior oesophageal part of Xiphinema index and from the base of the stylet knobs to the valve of the metacorpus in Ditylenchus dipsaci and Macroposihonia xenoplax.

    The oesophagus has one or more muscular swellings known as bulbs. These may be located near the mid length (designated as median bulb) or may be at the end of oesophagus (designated as posterior or cardiac or end bulb). According to the shape and position of the bulb, the oesophagi of the nematodes are divided into different categories: (1) Cylindrical-It is the most primitive muscular type and is encountered in marine nematodes of the family Enoplidae (for example Enopllts) or as in Mononchidae (Mononchus). From this basic type, other modifications in the structure of the oesophagi, possessing one or two bulbs, may be derived. (2) Dory-laimoid type—Anteriorly slender and non-muscular with a swollen muscular glandular posterior part, This type is encountered in the plant parasitic '"genera of Longidortts, Xiphinema, Trichodorus, etc. (3) Bulboid type—Slender anterior part ending with a posterior end bulb as in Ethmolaimus. (4) Rhabditiod type—Anteriorly, it has a wide region (corpus), usually leading to a median pseudobulb, followed by a narrow region (isthmus) and succeeded by an end bulb with valvular apparatus (as in "members of the genus Rkabditis),

(5) Diplogasteroid type—the anterior muscular region terminates in a median bulb. It is succeeded by a posterior glandular region form­ing a distinct bulb without a valvular apparatus (as in Dtphgaster).

(6) Tylenchoid type—Most of the plant parasitic nematodes belong to this type. It has a narrow oesophageal tube attached to the base of the stylet and is enclosed in a large thin walled tube. In most of the genera, there is a muscular median bulb with an ovoid valve (this bulb is much reduced or even absent in members of Neotylenchi-dae). The posterior part of the oesophagus is glandular and the

glands may form a distinct buccal bulb or a lobe overlapping the anterior part of the intestine. The duct of the dorsal oesophageal gland empties in the oesophageal tube wall, anterior to the median bulb, usually very near the base of the stylet. (7) Aphelenchoid type—It is similar to the tylenchoid type except for the fact that the dorsal oesophageal gland empties in the lumen of the oesophagus in the median bulb.

The oesophagus may be connected with the intestine with a structure termed as oesophago-intestinal valve which functions as an organ to impede the Sow back of food from intestine to the oesopha­gus. 1 his valve is cuticularly lined and may be very small or very large as well as conspicuous. It has been termed also as cardia. In some cases, as in Plectus, this valve may go deep in the intestinal lumen and may be wholly surrounded by the intestinal wall.

intestine. It is composed of a single layer of epithelial cells and is usually a straight tube in contrast to the reproductive organs, which may be reflexed or coiled. The number of the intestinal cells are extremely variable from relatively few in number to several thousand. In Tylenchida, the number is small, ranging from 16 to 24, while the Rhabditids have from 18 to 64. On the basis of number of intestinal cells, two nematode categories have been defined. When the number of cells are 128 or less, the nemato-des are categorized as oligocytous and when it is more than J28, the term polycytous is used. In oligocytous types, represented by plant parasitic group, the cells are large and multinucleate. They contain stored food materials like fats, glycogea. fatty acids and proteins, in addition to the protoplasm and nuclei.

Posteriorly, the intestine leads into the rectum which is lined by an invagination of body cuticle and opens at the anus. The rectal glands, three in number, open into the rectum, one dorsally and two subventrally. In male nematodes, the reproductive and the digestive systems join posteriorly in the ventral wall of the rectum to form the cloaca. The rectum is thus, wholly or in part, a cloaca in male nematodes. In both sexes, the rectum empties posteriorly through the anus aperture on the ventral surface of the body.