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Do nematodes have organ-level organisation?

Do nematodes have organ-level organisation?



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Some introductory biology books state that nematodes have a pseudocoelom. So, they have a false body cavity. So, does it also mean that they have organ level body organisation?


In Fig.1 is shown the body plan of a round worm:


Fig. 1. Nematode. source: University of Illinois

You ask whether there is an organization in terms of organs; an organ is:

A group of tissues that perform a specific function or group of functions;

A tissue is:

An aggregate of cells in an organism that have similar structure and function;

As you can see in Fig. 1, there are clearly organs present, including a mouth, pharynx, reproductive organs, among others. They fulfill different tasks and contain various tissues, each dedicated to specific functions. Hence, they fulfill the definitional requirement of being organs.


Adding to Christiaan's answer, I quote Caenorhabditis elegans' anatomy to substantiate that nematode have proper organ development.


The nematode body is cylindrical, elongated and smooth with no limbs protruding, such as is seen in the common garden worm though generally on a smaller scale. The body is contained within a tough elastic cuticle which in many species forms elaborate structures useful for identification. The presence of a cuticle is similar to the structure of arthropods, however unlike them the nematode cuticle is not chitinous but is comprised mainly of collagens. The cuticle is non-living, produced by cells of the epidermis in most of the worm, allowing it to grown between moults of the worm without the need for shedding, although this does occur a number of times during the development of most worms. It is permeable to allow ions and water to pass through and therefore plays a key role in maintaining the hydrostatic pressure, which in most nematodes is relatively high inside the worm. The cuticle also acts as an anchoring point during locomotion as a skeleton does in mammalian species.

Morphological differences in the cuticle are regularly used to identify different species of nematodes, though the functions of these are not all completely understood.

  • Annulations - Transverse lines in cuticle, possibly used as anchoring points in locomotion
  • Longitudinal Ridges - also known as 'synlophe', seen in some Trichostrongylidea species such as Nematodirus.
  • Alae or wings - Projections of the outer cuticle layer. Can appear either just at the anterior or posterior or along the entire length of the worm. In bursate males posterior alae for part of the copulatory bursa.
  • Spines - Protrusions of the cuticle on the surface of the nematode. Function unknown, could be in self defense or attachment to host.
  • Inflations - Vesicle like swellings of the cuticle function unknown. Found in Oesophagostomum species.

Phylum Nematoda: Habitat, Structure and Development

In this article we will discuss about Phylum Nematoda:- 1. Habit and Habitat of Phylum Nematoda 2. Structure of Phylum Nematoda 3. Body Wall 4. Body Cavity 5. Digestive System 6. Excretory System 7. Respiratory and Circulatory Systems 8. Nervous System 9. Reproductive System 10. Development.

  1. Habit and Habitat of Phylum Nematoda
  2. Structure of Phylum Nematoda
  3. Body Wall of Phylum Nematoda
  4. Body Cavity of Phylum Nematoda
  5. Digestive System of Phylum Nematoda
  6. Excretory System of Phylum Nematoda
  7. Respiratory and Circulatory Systems of Phylum Nematoda
  8. Nervous System of Phylum Nematoda
  9. Reproductive System of Phylum Nematoda
  10. Development of Phylum Nematoda

1. Habit and Habitat of Phylum Nematoda:

The Nematodes are popularly known as ‘Roundworms’ and sometimes called ‘Nemas’. They are among the most structur­ally simple of all worms because practically all of them depict materially the same basic body plan. Great number of nematodes is free-living and extends from north to South Pole and at the same time there exists a formidable array of parasitic forms living both on plants and animals.

In fact, every plant and metazoan animal has its quota of nematode parasites. The parasitic forms cause unimaginable damage to crop and domestic animals. So far as the absolute number of nematodes is concerned they are second to none than the insects and outnumber the insects in the variety of ecological nitches they occupy.

A survey of the abundance of nematodes in different sites gives the follow­ing figures:

(i) Roots of a single potato plant contain more than 40,000.

(ii) Intertidal muddy sand of sea has about 5 million/sq. metre.

(iii) Aerable land has up to 6 billion per acre.

Despite their superabundance in certain sites, the nematodes are never conspicuous and are not noticed for the reason that ma­jority of them are of microscopic sizes.

Free-living nematodes are saprozoic and feed on plant and animal remains. Some feed on yeast and bacteria. Few members prey on small protozoa and rotifers. Parasitic forms are provided with spines or teeth around the mouth which are used in piercing. None of the nematodes can engulf large particles. And in all essentialities they are microphagus or juice feeder.

The food habits of nematodes offer an opportunity to visualise evolution in action because with little imagination one can easily realise how saprophagus and her­bivorous forms have given rise to plant para­sites, and saprozoic types have been evolved into animal parasites.

The nematodes exhibit maximum tole­rance of environmental variations. They pos­sess the power to withstand extreme cold, heat and desiccation. The vinegar eel (Turbatrix aceti) living in vinegar (5% acetic acid) can thrive successfully up to a concen­tration of 14% acetic acid.

Living nematodes have emerged from mosses which have been rewetted after keeping them dried for about 5 years. The shelled eggs are much more resistant and remain viable for years. The eggs of Ascaris can withstand prolonged immersion in 12% formaldehyde, saturated solutions of mercuric chloride and in many toxic salts. Embryonic stages are usually less resistant.

2. Structure of Phylum Nematoda:

There exists a considerable similarity of organisation and shape in different nema­todes. General shape of the body as the name implies is round, cylindrical and tapering at both ends. The length usually varies from 0.4 m (Ascaris) to 1 m (Dracunculus).

The largest of all nematodes is Placentonema gigantissima. The females of this species attain a length of 8.5 m, the diameter being 2.5 cm and they parasitise the placenta of sperm whales. The females of all nematodes are generally larger than the male.

3. Body Wall of Phylum Nematoda:

On the outer surface of the body wall there is a cuticle which is hard and flexible. It is resistant to many solvents and gastric juices. Next to the cuticle lies the ectoderm. In some forms like ascaris the ectoderm is represented by a syncitial protoplasmic mass. Beneath the ectoderm only longitudinal muscles are found. The individual cells of the muscle fibres are very peculiar.

They are elongated and may reach a length of 10 mm. One end of the cell is contractile while the other end which houses the nucleus is non- contractile. The non-contractile part keeps contact with a nerve fibre. The longitudinal muscle layer is not continuous and is ar­ranged into four longitudinal bands. Two of these bands are dorso-lateral while the other two are ventrolateral in position.

In some free-living species the ectoderm bears unicellular glands. These glands help the animals to attach themselves to the sub­stratum.

4. Body Cavity of Phylum Nematoda:

Body cavity is not a true coelom because it is not lined by epithelial layer derived from mesoderm. Some workers have called it ‘Pseudocoelom’. According to them, the absence of mesenchyme in between the body wall and digestive tract has stood in a good way for the evolution of a more organised digestive system. The pseudocoelom is filled with a fluid and the fluid acts as a ‘hydro­static skeleton’.

5. Digestive System of Phylum Nematoda:

Digestive tract is complete. The mouth is situated at the anterior end of the body and remains surrounded by lips. In the basic plan there are six lips. But as seen in Ascaris the number of lips is reduced to three due to fusion. In some forms there may be many lips due to splitting. The mouth leads to a buccal capsule. The capsule is cuticular and the inner wall of the capsule in some cases forms plates.

The capsule may house three or more teeth. In some cases a hollow ‘Stylet’ is formed inside the capsule by the fusion of these teeth. The buccal capsule leads to the pharynx. The pharynx, like the buccal capsule is also cuticular.

The lumen of the pharynx is triangular. The pharyngeal wall is a syncytium of radial muscle fibres and the wall contains many one-celled glands. In some, the pharynx acts as a sucking appara­tus. Pharynx leads to the intestine.

Intestine is straight and is made up of a single layer of epithelium. Rectum is short and opens into the anus. The anal opening is on the ventral surface of the posterior end of the body. The anus is cuticular and in some forms like Ascaris it acts as a cloaca in males only.

The intestine is much reduced in Mermis. Feeding habits of nematodes are variable. Free forms may be herbivorous, carnivorous or saprophagous. Parasitic forms live on the nutrients inside the host’s intestine or in the blood and disintegrated tissues of the host.

6. Excretory System of Phylum Nematoda:

Excretory system of nematodes is very different from other animals as it does not show any phylogenetic relationship to the protonephridial system of platyhelminthes or to the excretory system of any other higher phylum. The pseudocoelom in primitive forms houses a very peculiar cell called ‘renette cell’. It is a glandular cell with a tubular neck.

In primitive forms a pair of such cells open to the exterior through the excretory pore situated on the ventral sur­face of the anterior end. It is believed that the prevailing type of excretory system in ad­vanced nematodes is an evolutionary out­come out of the primitive renette cell.

Bilat­eral arrangement of these cells together with tubular outgrowths from the cells has given rise to a ‘H’-shaped system in some interme­diate forms like Oxyurida, Ascarida.

In most advanced forms anterior elongation of the excretory tubule has been lost resulting an inverted ‘U’-shaped system. The evolution of ‘U’-shaped excretory system from renette cells is encountered during the embryonic development of many parasitic nematodes (Fig. 15.24).

7. Respiratory and Circulatory Systems of Phylum Nematoda:

There is no special organ or organ system for respiration and circulation. The cuticle serves as the respiratory surface. Few intes­tinal parasites like Ascaris can live on oxy­gen in young stage but in adult stage they get oxygen by anaerobic splitting of nutrient materials present inside the intestine of the host.

To send the end-products of digestion to the cells of the body wall and other parts, there is no special organ for circulation. End- products of digestion are absorbed by the intestinal epithelium and from there they are passed onto the fluid of the pseudocoelom. From the fluid of the pseudocoelom nutrient materials reach the cells of the body wall.

8. Nervous System of Phylum Nematoda:

The nervous system is of simple type and consists of a ‘brain’ or nerve ring from which nerves extend to the anterior and posterior parts of the body. The nerve ring is present round the pharynx and is formed by two lateral pairs of ganglia.

From the ganglion a ventral nerve cord extends along the mid- ventral line and ends in a ganglion above the anus. Dorsal motor nerve and three pairs of lateral sensory nerves are also present.

9. Reproductive System of Phylum Nematoda:

In nematodes, sexes are separate. Adult males are smaller in size than the females, and in most males the posterior end of the body is curved. Male reproductive system consists of a single thread-like much coiled structure. The anterior part of the coil forms the testis, middle part forms the vas defer­ens and posterior part forms the seminal vesicle.

The testis may be monorchic (i.e. single testis, e.g., Ascaris) or diorchic means two testes when present in male reproduc­tive system in nematodes. The seminal vesi­cle continues as the ejaculatory duct and opens into the anus.

Inside the anus there is a pocket which contains a pair of eversible penial spicules. That means there is no male gonopore. The sperms are cone-shaped and have a broad base and a tapering apex. The sperms show amoeboid movement inside the body of the female.

The female reproductive structures con­sist of a pair of ovaries, a pair of oviducts and a pair of uteri. The two uteri unite to form a vagina which opens to the outside by a single female genital aperture situated on the ventral surface of the body.

If there is one tract containing single ovary, oviduct and uterus called monodelphic but didelphic and polydelphic also occur. In Ascaris there are two tracts containing paired ovaries, oviducts and uteri. In Trichinella, the female reproductive structure is single.

10. Development of Phylum Nematoda:

Eggs are fertilized in the vagina of the female. Soon after fertilization the eggs be­come enveloped by three membranes—an outer albuminous covering, a middle chori­onic covering of chitinous nature and an inner vitelling membrane. In hook-worms the outermost layer is absent. Cleavage is of determinate type.

In Ascaris the first cleav­age divides the egg into a somatic cell and a germ cell. Blastula is a coeloblastula. Epiboly is the usual mode of gastrulation. In Ascaris and Trichuris fertilized eggs leave the body of the mother and host before segmen­tation.

Segmentation starts outside the body and later on it becomes infective. In Enterobius the eggs leave the body of the mother and host in segmented condition. In Ancylostoma, the eggs leave the body of the mother and host in partially segmented condition.


Class Rhabditea is composed of both parasitic and free-living nematodes. However, the majority of nematodes that exist as parasites are found in this class.

Parasitic nematodes found in class Rhabditea include Ascaris, Enterobius (e.g. human pinworm), Necator species as well as Wuchereria species. These species infect and cause a variety of diseases in human beings ranging from minor to very serious conditions.

For instance, Wuchereria bancrofti, a species belonging to the genus Wuchereria causes a serious condition known as Lymphatic filariasis characterized by the swelling of arms and legs. This is a serious condition that is not only painful, but also causes the disfiguration of arms and/or legs.

Compared to other worms in this class that can be found in many regions across the world, this species of nematodes are common in the tropics. On the other hand, intestinal infections caused by such nematodes as Enterobius and Ascaris can be treated and do not usually result in serious health problems.

Free-Living Nematodes in Class Rhabditea

One of the best examples of free-living nematodes in class Rhabditea are members of the genus Caenorhabditis.

Caenorhabditis elegans, a species of Caenorhabditis is a small, free-living worm found in temperate environments. Apart from nutrition sources in decaying matter, these worms also obtain nutrients from rotting fruits. In particular, Caenorhabditis have been shown to live in bacteria-rich habitats such as compost where they obtain their nutrients and only depend on other organisms (insects etc) for transport from one location to another.


Is a cnidaria a Protostome or Deuterostome?

Click to read in-depth answer. Accordingly, is nematoda a Protostome or Deuterostome?

The two clades diverged about 600 million years ago. Protostomes evolved into over a million species alive today, compared to about 60,000 deuterostome species. Protostomes are divided into the Ecdysozoa, e.g. arthropods, nematodes the Spiralia, e.g. molluscs, annelids, platyhelminths, and rotifers.

Subsequently, question is, are annelids Protostomes or Deuterostomes? Annelids are members of the protostomes, one of the two major superphyla of bilaterian animals &ndash the other is the deuterostomes, which includes vertebrates. Arthropods are now regarded as members of the Ecdysozoa ("animals that molt"), along with some phyla that are unsegmented.

Furthermore, are jellyfish Protostomes or Deuterostomes?

Origins and evolution. The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes. Chordates (which include all the vertebrates) are deuterostomes. It seems likely that the 555 million year old Kimberella was a member of the protostomes.


Class Enoplea

Enoplea, like Chromadorea, also makes up the phylum Nematoda. However, compared to Chromadorea, researchers have described Enopleans as ancestrally diverged nematodes.

As such, they are the more ancestral group of nematodes that have not diverged as much as members of the other classes. Some of the worms belonging to this class include Trichuris, Diotyphyme, and Diotyphyme.

Also known as the human whipworm, Trichuris trichula are roundworms that are responsible for trichuriasis.

Apart from being human parasites, Trichuris trichiura have the following characteristics:

  • Female T. trichiura can produce as many as 20,000 eggs per day which enter the infective stage after two or three weeks. Eggs of T. trichura are characterized by a prominent bipolar plug
  • Pinkish-white in color
  • Their anterior esophageal end is narrow in appearance while the posterior end is thicker
  • Obtain their nutrients from tissue secretions
  • Measure between 35 and 50 mm in length
  • Mostly found in the tropics (particularly Asia)

Some of the other worms affect other types of animals. For instance, Trichinella spp. infect Black Bear, Dingo, and Polar Bears among others. As well, some members of Dioctophyme can affect both human beings and various carnivores and survive as parasites.

Compared to other classes of nematodes, Enopleans have the following general characteristics:

  • Possess a cylindrical esophagus. For some of the species, however, the esophagus is bottle-shaped
  • They have amphid (well-developed Amphids) of Enoplea resembling a pocket. Whereas the Amphids (innerved invaginations among nematode species) of the members of chromadorean may be slit, pore or spiral-like. Enopleans are pocket like which may be used to differentiate between the two classes.
  • Compared to chromadorean species, Enopleans are also characterized by a smooth appearance or fine lines on the surface of some of the species
  • Possess a simple excretory system that is lacking lateral canals (also made up of one or a few ventral or glandular cells)
  • Lack phasmids

Physiological Processes of Nematodes

In nematodes, the excretory system is not specialized. Nitrogenous wastes are removed by diffusion. In marine nematodes, regulation of water and salt is achieved by specialized glands that remove unwanted ions while maintaining internal body fluid concentrations.

Most nematodes have four nerve cords that run along the length of the body on the top, bottom, and sides. The nerve cords fuse in a ring around the pharynx, to form a head ganglion or “brain” of the worm, as well as at the posterior end to form the tail ganglion. Beneath the epidermis lies a layer of longitudinal muscles that permits only side-to-side, wave-like undulation of the body.

View this video to see nematodes move about and feed on bacteria.

Nematodes employ a diversity of sexual reproductive strategies depending on the species they may be monoecious, dioecious (separate sexes), or may reproduce asexually by parthenogenesis. Caenorhabditis elegans is nearly unique among animals in having both self-fertilizing hermaphrodites and a male sex that can mate with the hermaphrodite.


Scientists map the brain of a nematode worm

Researchers have mapped the physical organization of the brain of a microscopic soil-living nematode worm called Caenorhabditis elegans, creating a new model for the architecture of the animal's brain and how it processes information.

In a surprise twist, they found a large degree of variation in the structure of some neural circuits or pathways in individual worms which complemented a core set of neural circuits common to different animals.

The scientists say the worms' brains might have a lot more in common with larger animals than previously thought.

Created by neuroscientists at the University of Leeds in collaboration with researchers in New York's Albert Einstein College of Medicine, the brain map reveals that different spatial regions support different specialised circuits for routing information in the brain, where information is integrated before being acted upon.

The study is published today (24 Feb) in the scientific journal Nature.

C. elegans are nematodes that feed on bacteria found in rotting vegetation in your garden. They are only around a millimetre in length and as thin as a human hair.

An adult worm has exactly 302 cells in its nervous system -- by comparison, the human brain has around 100 billion cells. But almost two thirds of the worm's nerve cells form a ring in the head region, where they make thousands of connections with each other.

This 'brain' is the control centre of the animal, where much of the sensing and decision-making takes place.

Even though the brain is very compact, the animal displays a range of complex behaviours, and neuroscientists have been interested in understanding its brain for decades. Previous studies have created 'wiring diagrams' for the connections between nerve cells.

This latest study, though, is the first to provide the complete spatial coordinates to those circuit diagrams.

Professor Netta Cohen, Computational Neuroscientist at the University of Leeds, who supervised the research, said: "The brain needs to organise information flow to control the animal's behaviour. But how the structure and function of the brain are related is an open question. Providing the spatial representation of the circuitry has allowed us to uncover the modular structure of this animal's brain."

Creating the brain map

The researchers used a legacy collection of electron microscope images of the brain of an adult and juvenile nematode worm. Those images revealed individual brain cells or neurons, allowing the researchers to map the organisation of the worms' neural circuits, from the level of individual cells through to the large scale architecture of the entire brain.

Structure-function of the brain

The scientists identified known neural circuits and pathways within the brain such as a navigation neural circuit which an animal would use to follow smells and tastes to forage for food. Another circuit is thought to facilitate mechano-sensation, so it would feel its way as it wriggles through the soil -- or sense if it is surrounded by bacteria.

Their theory is that information is processed in the worm's brain through a number of 'layers'. In fact, a similar layered architecture is found in the human brain. Information flow starts in sensory cells, which respond to the environment. For example, cells may sense bacteria but are they the right bacteria to feed on -- do they smell like the 'right' bacteria? The answer requires information to be integrated from multiple senses before being sent to the command area of the brain for action.

Professor Cohen said: "The brain map reveals a very elegant structure to support information flow through a worm's brain and it is more sophisticated than the traditional view that simple animals follow a stimulus-response path.

"The map suggests a convergence of different neural circuits -- and this allows the worm to integrate all of the different cues it is receiving through its sensory cells and to coordinate the response."

Variation in brain structure

During their study, the researchers were surprised to discover the extent of individual variation in the worms' brains.

C elegans is one of the most studied animals in biology. During the life of the worm, the way its cells divide and grow follows a strict blueprint which is observed across the entire species. But when it comes to the brain cells, there seemed to be a high degree of variation in the way the brain cells formed contacts with neighbouring cells to create neural circuits.

Using mathematical and computer models, the scientists were able to discern between those connections that are likely to form the 'core' circuit across a large population of animals, and those that appear to be variable between individuals.

Dr. Christopher Brittin, a former PhD student at the University of Leeds and first author on the paper said: "This work raises interesting questions about how even seemingly simple nervous systems are able to accommodate both core and individualized brain circuitry."

The scientists found that only around half the wiring in the worms' brains is similar -- the other half showed variation.

Professor Cohen added: "This finding was really exciting for us. First, this suggests that worm brains have a lot more in common with the brains of higher animals than we knew or expected, and the lessons learned about worms can help us learn about brains more generally."

The variable connectivity may support individuality, redundancy and adaptability of brains as the animals face challenging, dangerous and ever-changing environments.


Nematode

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Nematode, also called roundworm, any worm of the phylum Nematoda. Nematodes are among the most abundant animals on Earth. They occur as parasites in animals and plants or as free-living forms in soil, fresh water, marine environments, and even such unusual places as vinegar, beer malts, and water-filled cracks deep within Earth’s crust. The number of named species is about 20,000, but it is probable that only a small proportion of the free-living forms have been identified. A great deal of research has been conducted on the parasitic forms because most of them have some medical, veterinary, or economic importance.

Nematodes are bilaterally symmetrical, elongate, and usually tapered at both ends. Some species possess a pseudocoel, a fluid-filled body cavity between the digestive tract and the body wall. Like arthropods and members of six other phyla, nematodes secrete an external cuticle that is periodically molted. These animals have been provisionally grouped together as the Ecdysozoa, a taxonomic category based on the assumption that molting has evolved only once. So far, gene sequence data from several molecules support such an assumption.


The Locomotion of Nematodes

1. The form and frequency of the waves passing down the bodies of small freeliving nematodes (Panagrellus, Rhabditis and Turbatrix) depend on the nature of the external medium.

2. Observations of animals moving in such media as syrup, agar gels, and dense suspensions of particles suggest that the relationship between the speed of progression of the animal to the speed of propagation of the waves along the body, depends on the relative resistance exerted by the medium to displacement of the body in directions normal to and tangential to its own surface.

3. When swimming in water the body of a nematode yaws periodically in a transverse plane and the axis of progression does not coincide with that of the waves the displacement of an element of the body relative to the medium depends on its position on the body. The envelope of one complete wave exhibits two characteristic nodes.

4. A suspended particle originally situated near the anterior end of a swimming animal moves tangentially backwards along the surface of the body, but the speed at which it does so is not constant and is always much less than that at which the waves travel relative to the ground. The average speed of a particle close to the surface of the body is about one quarter of that of the waves.

5. When plotted relative to the ground the path of displacement of a particle exhibits characteristic loops. When viewed from above and in the direction of the animal's progression, all particles on the left side of the body traverse their loops in a clockwise direction all those on the right side move anticlockwise.

6. There is a highly characteristic pattern of circulation round the body of the animal. Water in the vicinity of a wave crest moves in the opposite direction to that of the propagation of the waves, but in the same direction as the waves when situated in a wave trough. The circulation extends for a considerable distance from the surface of the body.

7. The flow round the body of a swimming nematode is essentially the same as that in the neighbourhood of an undulating sheet of rubber. Its analysis presents an interesting but difficult hydrodynamical problem.


Watch the video: The Fungus That Traps and Kills Nematodes (August 2022).