All nematodes possess a similar digestive system or tract that consists of a mouth, sometimes reffered to as a stoma, the pharynx or oesophagus, and the intestine. While present in all nematodes, there is some room for variation of these structures besed on the feeding habits of the different species. The most important types of nematode feeding or trophic groups include:
Algivores:- Nematodes found in aquatic environments and rarely in soil that feed primarily on algae.
Herbivores:- Plant parasitic nematodes comprise this classification. They all have modified mouth structures and use a ''stylet'' or needle like protrusion to pierce cells during feeding . This parasites can be divided into either Ectoparasites and Endoparasites (Those that stay in the soil and those that enter the plant through the root ). The stylet can be seen in the herbivore mouthpart diagram provided Bacterivores. Those free living roundworms whose primary source of food is bacteria; they help in decomposing organic matter. Bacterivore mouthparts reflect their food source in the fairly open space where soil and other organic matter is taken in.
Bacterivores:-Some times of free-living nematodes can only feed on bacteria. These types of nematodes have a "mouth", or stoma, which is hollow tube for swallow the bacteria (order Rhabditida) . These nematodes are also beneficial in the decomposition of organic matter.
Fungivores:- A group of nematodes that uses a “style to puncture fungal hyphae,” thus feeding mainly on fungi; they are also important in decomposition, (order Aphelenchida).
Predators:- Nematodes whose primary source of food is small animals of comparable size and other soil nematodes of similar size; they are less common than most other types of nematodes. They feed on both plant parasitic and free-living nematodes(order Mononchida ).
Omnivores:- A few types of nematodes may be able to feed on several different types of food and are thus considered omnivores. For example, in some cases nematodes may ingest fungal spores as well as bacteria(order Dorylaimida).
Animal Parasites:- Those nematodes that inhabit animal hosts in their tissues, intestinal tracts, and other organs and derive their nutrition from the host.
Unknown:- Due to the extremely limited study of nematoA few kinds of nematodes may feed on more than one type of food material, and therefore are considered omnivores. For example, some nematodes may ingest fungal spores as well as bacteria. Some members of the order Dorylaimida may feed on fungi, algae, and other animals. des, there are probably many different food sources and feeding habits that have not yet been discovered .
Digestive system :-
The majority of nematodes possess a wall developed alimentary tract composed of three regions the stomodaeum. The nematode digestive tract can be divided into an ectodermal foregut comprising the pharynx, and if present the buccal cavity, endodermal middle intestine and ectodermal posterior intestine.
Stoma:- The most anterior portion of the alimentary tract is the mouth, which is commonly referred to as the stoma. These structures are complex and variable, reflecting the different feeding habits of the species concerned. The stoma is composed of several sections. Each section includes a ring of cuticle surrounding a portion of the mouth cavity. These portion of the stoma starting from the mouth opening , are cheilostom, prostom, mesostome, metastome and telostome. They are surrounding by the corresponding rhabdions, cheilorhabdion, prorhabdion, mesorhabdion, metarhabdion and telorhabdion.
Buccal cavity:- The mouth of the nematodes opens into a buccal cavity which is somewhat tubular and lined with cuticle. The cuticular surface is often strengthened with ridges, rods, or plates or it may bear a large number of teeth .Teeth in carnivorous nematodes may be small and numerous or limited to a few large, jaw like process in some carnivorous as well as in many species that feed on the contents of plant cells.
Pharynx(Oesophagus):- The buccal cavity leads into a tubular pharynx, referred to as oesophagus by nematologists. The pharyngeal lumen is triradiate in cross section and lined with cuticle. Some species possess valves that prevent the oesophagus empting to the outside or filling from the intestine .
The opening and closing of these valves is co-ordinated with the pumping cycle. In the absence of valves, a wave of dilation passes down the oesophagus so that only half of its length is dilated at any one
time. This provides a self-sealing system.
Excretory sysetem:- In nematodes excretory system sometimes are glandular and sometimes are tubular. The glandular system consists of a large cell (single), the ventral gland and they open to the exterior by duct. The tubular type consists of two lateral ducts which are present next to the lateral cords, and connected by transverse ducts to a cuticle lined median duct or ampula which end to a excretory pore.
Bennet-Cark's Model :-
The oesophagus has a triradiate cuticle lined lumen which is connected to the outer wall by radial muscles. The oesophagus can be considered as two concentric cylinders(the outer wall and the lining of the lumen) which are connected by the radial muscles. Such a cylinder increase in diameter more easily than in length. When the radial muscles contract, there is an increase in pressure within the cylinder. If this pressure is less than that within the pseudocoel the excess pressure prevents the cylinder increasing in diameter and it therefore lengthens and becomes thinner, keeping the lumen closed. As the radial muscles continue to contract the pressure within the cylinder increases until it is greater than in the pseudocoel. The cylinder then increases in diameter and minimised stresses by becoming shorter and wider, resulting in the opening of the lumen.
When the radial muscles relax, or if the pressure within the cylinder in decreased by the opening of the oesophagal -intestinal valve, the process is reversed. This, together with the elasticity of the circular lining, closes the lumen.
Digestion and absorption:-
They usually take place within the intestine. The intestine is divisible into three regions, the anterior ventricular region, the mid-intestine or the intestine proper and a posterior prerectal region. None of these regions is lined with cuticle, but the outer wall is covered by a basal membrane.
Although the intestine is generally straight and cylindrical, some nematodes have intestinal outgrowth or cecae which may be source of digestive enzymes or a place for harbouring symbionts. The anterior ventricular region generally contains a few cells which are thought to be glandular in function. The mid-intestine is formed by a single layer of cells that may be uni or polynucleated or form a syncytium.
The intestinal lumen is lined by the cells with microvillae, which vary considerably in size and shape from species to species. In addition to absorbing and metabolizing nutrients, the intestinal cells serve as food storage areas. Food is stored as glycogen, lipid, and protein. A number of digestive enzymes, including proteases, lipases and amylases have been found in intestine.
Defecation:- A high internal turger pressure provides the force for defecation. Indeed, liquid faeces
may be ejected for some considerable distance upon the opening of the anus. An intestinal-rectal valve controls the entry of the materials in to the rectum.
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