Nematode Structure

General introduction to marine nematodes

The Phylum Nematoda consists of small multicellular vermiform organisms which can be found in almost every conceivable environment. Apart from existing as free living forms in soil, freshwater and marine benthic habitats, they also parasitise plants and other animals, including man. The whole phylum currently contains some 20,000 nominal species. About 4000 species are free living marine forms and, of these, some 450 representing 170 genera have so far been reported from the British Isles (here taken to include Ireland).

At the microscopic level, marine nematodes are quite varied morphologically and no one species can be considered representative. Therefore, the following description of the general structure is illustrated with a generalised nematode. Press each button below for diagrams.

 

 

Most adult marine nematodes are elongated cylindrical worms around 1 - 2mm long, although some of the larger forms such as those found in kelp holdfasts may be several millimetres in length. Fortunately, the vast majority are sufficiently transparent to allow their internal anatomy to be seen without recourse to special methods of preparation and this considerably increases the number of characters available for practical identification purposes.

The body is essentially a tube within a tube. The external tube is the body wall consisting externally of a cuticle layer and internally of a longitudinal muscle layer. Nematodes do not possess circular muscle so are unable to elongate the body. Movement is brought about by alternate contractions of the dorsal and ventral muscle blocks (Fig. 1C, D) working against the hydrostatic skeleton provided by having a high internal turgor pressure. This results in the highly characteristic serpentine pattern of locomotion. Because the flexing is in the dorso-ventral plane, the fixed nematode normally comes to lie on a microscope slide with a lateral side uppermost, which is why most illustrations are of lateral views.

The internal tube is the gut which is terminal at the anterior but subterminal posteriorly, so that nematodes have a tail. The gut is differentiated into a buccal cavity, a muscular oesophagus (more accurately termed a pharynx), an intestine and a short rectum. Between gut and body wall lies a fluid-filled cavity called a pseudocoelom (so called because it appears to lack an epithelial lining) in which the reproductive organs are to be found.

The body wall and gut provide many characters useful in identification together with taxonomically useful parts of the nervous, excretory and reproductive systems. The four characters used in the nematode genera identification key are the buccal cavity (mouth), the tail shape, the amphids, and the cuticle pattern. These and the other features will now be described in more detail.

 

Buccal cavity

The buccal cavity exhibits a great variety of form, reflecting the great range in feeding methods among marine nematodes. The buccal cavity may be absent or minute (Fig. 2A) to spacious but unarmed (Fig. 2B). Many species have buccal cavities armed with immoveable projections of the wall, referred to as teeth (Fig. 2C) or moveable structures termed mandibles or jaws (Fig. 2D). In addition there may be rows of small denticles lining the buccal cavity, or there may be other projections. The detailed form of the buccal cavity is one of the most important characters for species identification. Press each button for diagrams.

 

 

Tail

The tail shape can be highly characteristic, the main themes around which there are variations being round, conical, conico-cylindrical or elongated (Fig. 3). The length of the tail is often quoted in terms of the anal (or cloacal) body diameter (a.b.d., Fig. 4). The tail contains the unicellular caudal glands, usually three (Fig. 1), which may be confined entirely to the tail or extend anterior to the anus/cloaca. The caudal glands are responsible for secreting an adhesive which exits via a specialised structure in the tail tip called a spinneret. Certain species may lack caudal glands entirely. Press each button for diagrams.

 

 

Amphids

Nematodes have two amphids which are specialised sensilla situated laterally on the head. These bilaterally symmetrical structures are usually of major importance for identification. Basically there are two kinds, spiral or non-spiral. Usually the non-spiral amphid is a pocket-like structure (Fig. 4A) where the external opening (aperture) is in the form of a transverse slit leading to a cavity (fovea) filled with a gelatinous substance (corpus gelatum). In the spiral amphid the fovea is open and elongated, normally turning ventrally (from the nerve) although in a few cases the amphid may be dorsally wound. When the fovea makes just one turn the amphid may be termed loop-shaped (Fig. 4B) while if several turns are made it is multispiralled (Fig. 4D). Variations on this theme include the transverse amphid (Fig. 4E) and the circular amphid (Fig. 4C), but all are essentially spiral.

 

 

In species descriptions, the amphid width is often quoted as a proportion or percentage of the corresponding body diameter (c.d.) and the distance from the anterior of the head given in terms of head diameters (h.d.) to the anterior amphid margin (Fig. 4B).

 

Cuticle

The cuticle may appear to be entirely smooth or to be transversely annulated, the latter usually being called a striated cuticle. In many groups, there may also appear to be dots (punctation) which can be arranged irregularly or in rows. These and various elaborations of the cuticle may be confined to the lateral parts of the cuticle only, when it is termed lateral differentiation. Examples of cuticle patterns are shown in figure 5. Where the cuticle pattern is the same in all regions of the body it is sometimes referred to as homogeneous; where the pattern changes along the length of the body it is called heterogeneous ornamentation. In addition to dots there may also be pores in the cuticle laterally.

 

Other sense organs

The cuticle bears a number of sensory structures, collectively called sensilla (singular: sensillum) which, although having an underlying basic structure, may take a variety of forms. Long hair-like sensilla are called setae (singular: seta) while if only nipple-like they are called papillae (singular: papilla).
Sensilla found on the general body surface (somatic sensilla) may be arranged in definite longitudinal rows or apparently distributed at random. Sensilla on the tail may be longer or stouter than those on the rest of the body, especially in the male, when they may be differentiated as caudal setae or spines. Specialised sensilla at the tail tip are called terminal setae. The region between the head and the base of the oesophagus is often called the cervical (neck) region, so that setae here may be identified as cervical setae.

The arrangement of the sensilla on the head is rather specialised and conforms to a characteristic basic pattern, which is generally considered to be the primitive arrangement (Fig. 6A). Six labial (lip) sensilla (two lateral and four submedian) surround the stoma (mouth opening) and are normally papilliform, or only short setae, making them difficult to detect. Posterior to the labial sensilla there is normally a circle of six (two lateral and four submedian)and a circle of four (submedian) sensilla, usually setiform: anterior and posterior cephalic setae respectively. This basic arrangement can be denoted by the abbreviation 6 + 6 + 4 (Fig. 6B). However, the anterior cephalic sensilla may be papilliform, so that there appears to be only four setae (Fig. 6C) or the anterior and posterior circles may be situated at the same level, the latter arrangement denoted by the formula 6 + 10 (Fig. 6D). In several groups additional setae may be found associated with the basic ten cephalic setae. The lengths of these setae are often quoted in the literature as a proportion of the diameter of the body taken level with the posterior cephalic setae, the head diameter (Fig. 6D).

 

 

Alimentary canal

The buccal cavity leads to the oesophagus, which is the muscular anterior part of the gut responsible for pumping food into the intestine. The lumen of the oesophagus is tri-radiate in cross-section (Fig. 1C). The oesophagus may be cylindrical throughout (Fig. 7A) or have a posterior bulb complete with a valve structure (Fig. 7B). Several nematodes have paired pigment spots or true ocelli (i.e. having a lens-like structure) situated laterally or dorso-laterally on, or partly inside, the anterior part of the oesophagus (Fig. 7).

 

 

About half-way along the length of the oesophagus lies the circumoesophageal commisure or nerve ring, which is usually the only part of the nervous system detectable with ease. Its relative position may in some cases be of use in identification. The so-called excretory system, which in reality is of unproven function, consists of a single ventral gland (sometimes referred to as a renette cell) and a duct extending anteriorly to open by a ventral pore somewhere in the oesophageal region (Fig. 7A). The exact position of the pore, or its position in relation to the nerve ring or buccal cavity, is occasionally of taxonomic use. The muscular structure sometimes found at the base of the oesophagus and opening into the intestine is the oesophagointestinal valve, usually referred to more simply as the cardia. The intestine itself is a straight tube formed of a single layer of cells and is taxonomically unimportant. The rectum connects the intestine to the anus in females or cloaca in males, and may have conspicuous muscles attached to the dorsal wall for opening the passage against the internal turgor pressure.

 

Reproductive system

The female may have one or two ovaries (monodelphic/ didelphic) which may or may not be reflexed (Fig. 8A, B). The number and structure of the overies is important and can be used to distinguish major taxonomic groups. The position of the vulva is usually at about the middle of the body in didelphic nematodes but can be closer to the anus in monodelphic forms; its position from the anterior is given as a percentage of the total body length (V %), including the tail unless the latter is very long, when a modified percentage of anterior-to-anus length (V' %) should be given.
The male usually has two testes (diorchic) which may be opposed (Fig. 8C) or in tandem (Fig. 8D) but some major groups have only one (Fig. 8E). However, gonads are often difficult to distinguish in practice and are usually of little importance for species identification.

 

 

The most important parts of the male system for practical purposes are the cuticularised copulatory structures and accessory organs. Typically there is a pair of cuticularised spicules and a guiding piece called a gubernaculum which lie in a sac opening into the dorsal side of the cloaca (Fig. 1A). However, in practice, the detailed arrangement of the vas deferens, rectum and spicule sac around the cloaca are difficult to elucidate and are rarely depicted. The shape and size of the spicules and gubernaculum vary considerably and are normally of great value in identification. Likewise, the presence and structure of the various kinds of precloacal supplements can be highly important.

 

Examination

Examination and identification requires a reasonably good quality microscope equipped with a 100 oil immersion lens. Without this commitment to microscopy, frustration is guaranteed. The availability of an interference-contrast microscope is also an advantage although phase-contrast is of little use. A drawing-tube or camera-lucida will be found of considerable help.
To observe the different characteristic features of a nematode it must be remembered that the animal is essentially a three dimensional cylinder yet the image visible through the microscope is two dimensional. Minute changes in focussing distance allow observation of the different features within different planes of focus passing from the upper surface of the nematode, through the different levels of the interior of the nematode to its lower surface.

When attempting to identify specimens, it is often more convenient to work from a drawing. Each species encountered should be drawn fairly accurately; because the various features are usually in different planes of focus, it is only in the drawing that their relative positions and sizes can be visualised. The aim should be to have at least one good drawing of a male head, tail and copulatory apparatus. If the first specimen of a species encountered is a juvenile or a female, then this should be sketched with a view to replacing it subsequently if and when a male turns up. Pencil drawings of a standard needed to help identify a species from a taxonomic description in the primary literature should not take long and will provide an accurate and valuable record.

Measurements from drawings or simple outlines can be taken using either dividers for straight lines or a good quality map-measurer for curves. Appropriate calibration from a stage micrometer is then applied. Alternatively, an eyepiece micrometer can be used directly for straight measurements. Solid curved structures such as spicules are usually measured as the chord rather than the arc. The typical measurements taken are given in Table 1: proforma lists such as these can prove very useful. In descriptions, an indication of the relative diameter of the body, oesophagus length and tail length are often given by the De Man ratios 'a', 'b' and 'c', which are the ratios of the total body length to maximum body diameter (usually in the middle of the body), oesophagus length and tail length respectively. Fig. 9 gives examples of the appearance of nematodes when these ratios are varied.

After the investigation is completed, and if the results are to be published, it is strongly recommended that voucher specimens of all the putative species are deposited in an appropriate institution with good curation facilities; in the United Kingdom this would be the Natural History Museum in London, which will store the material safely and issue the collection with a registration number. There can be no blame attached to an honest misidentification but good scientific practice would be to provide the material evidence for such a possibility to be rectified.

 

Click here to view a sample proforma list used when recording taxonomic data (HTML format).

 

Click here to download a sample proforma list used when recording numerical and non-numerical taxonomic data (Word 97 format)













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