Phylum Aschelminthes: Characteristics, Examples, Platyhelminthes, Classification, Topics

Phylum Aschelminthes

Phylum Aschelminthes includes a group of worm-like invertebrates, otherwise called nematodes or roundworms, which are characteristically cylindrical with unsegmented bodies. Historically, the phylum Aschelminthes was considered a single phylum, but it became further divided into multiple phyla, with the nematodes being the best-studied ones. They were known and observed for centuries, but until recent decades, reasonably major forward steps were made in understanding their biology. Aschelminthes occupy quite an important place in the biological hierarchy since they are widely spread, perform some ecological functions, and have an impact on human health and agriculture.

Phylum Aschelminthes Overview

The phylum Aschelminthes includes bilaterally symmetrical, triploblastic, pseudocoelomate animals. These are commonly known as roundworms and possess an organ-system level organisation.

Classification of Aschelminthes

Aschelminthes are pseudocoelomate and invertebrate animals that are characterised by a simple body plan and a pseudocoelom. The differences among these classes are quite distinct.

Class Nematoda

• Nematodes are unsegmented, cylindrical worms covered by a tough, flexible cuticle that they moult periodically.

• They have a complete digestive system with a distinct mouth and anus.

• They have a simple nervous system in that they do not have a brain but have a ring of nervous tissue around their pharynx with longitudinal nerve cords that run down their length.

• Nematodes have a sexual mode of reproduction, which means most species have separate male and female reproductive parts. The fertilisation is internal.

• They are highly diversified and are found in almost every habitat, which includes soil, freshwater, and marine environments, and as parasites in the plant and animal bodies.

Examples

• Ascaris lumbricoides: This is the most common nematode, or roundworm, that parasitises the intestines of humans.

• Ancylostoma duodenale: Known as hookworm, it is responsible for anaemia and several other serious disorders.

• Enterobius vermicularis: The pinworm, a nematode worm, lives in the intestines of children.

Class Rotifera

• The rotifers are microscopic, aquatic animals that have their body divided into a head, trunk, and foot.

• The head contains a rotating, ciliated organ, called the corona, used for locomotion and food capture.

• They have a complete digestive system that contains a mastax, which is a unique pharynx provided with jaws to grind up the food.

• Most rotifers are parthenogenetic, though some have sexual reproduction.

• Mainly found in freshwater, some species inhabit marine and damp terrestrial habitats.

Examples

• Brachionus plicatilis: A common rotifer species used in aquaculture.

• Philodina roseola: A freshwater rotifer often used in scientific studies.

Class Gastrotricha

• Gastrotrichs are tiny, worm-like, flattened animals covered with cilia, which assist in locomotion and nourishment.

• They have a simple digestive system with a mouth, pharynx, and intestine.

• They have sexual and asexual ways of reproduction through parthenogenesis. Many of them are hermaphrodites.

• Gastrotrichs are mainly inhabitants of freshwater and marine sediments, on algae.

Examples

• Chaetonotus: A genus of freshwater gastrotrichs.

• Lepidodermella: Another genus commonly found in freshwater habitats.

Class Kinorhyncha

• Kinorhynchs are otherwise known as mud dragons.

• Their body is segmented, having a head, neck, and trunk. The body is adequately plated and spined.

• They can move around by extending and retracting their head.

• They get their food from microorganisms and organic matter in the sediment.

• It has sexual reproduction with two distinct sexes.

• Mostly found in sediments of marine environments.

Difference Between Platyhelminthes and Aschelminthes

Both Platyhelminthes and Aschelminthes are two important groups of invertebrate animals included in the Animal Kingdom. Although both of these groups contain simple worm-like organisms, they are different in body structure, body cavity, digestive systems, etc.

Phylum Aschelminthes Characteristics

Members of Phylum Aschelminthes (also known as Nematoda) are bilaterally symmetrical, unsegmented, and pseudocoelomate organisms with a complete digestive tract and a tough outer cuticle.

• The Aschelminthes are bilaterally symmetrical, which means they have an elongate, basically cylindrical body, mostly tapering at both ends and are covered with a tough, non-living cuticle.

• They develop bilateral symmetry, wherein there is the presence of a definite head and tail region.

• They possess a well-developed digestive system with a definite mouth and anus.

• Reproduction in most of them is sexual. They are dioecious, though hermaphroditism is also found.

• The habitats of Aschelminthes range from soil to freshwater, marine environments, and even as parasites in plants and animals.

• Their life cycle ranges from simple direct development to highly complex cycles involving multiple hosts.

Aschelminthes Diagram

The diagram given below shows the internal structure of the phylum Aschelminthes. They are triploblastic, having 3 germ layers- ectoderm, mesoderm and endoderm.

Morphology and Anatomy of Aschelminthes

These organisms have a cylindrical, elongated body tapered at both ends. They possess a straight alimentary canal, a pseudocoelomic cavity, and separate sexes (dioecious). Males are generally smaller than females and often possess curved posterior ends.

External Morphology

• The size of the nematodes can vary from microscopic to a few meters. Their body shape is cylindrical, normally tapered at both ends.

• It has a flexible, non-living cuticle that periodically undergoes moulting. There are also surface structures that may comprise sensory bristles or papillae.

Internal Anatomy

• A complete digestive system will have a tubular gut extending from mouth to anus.

• A relatively simple nervous system, wherein a nerve ring is found in the region of the pharynx; also, longitudinal nerve cords extend posteriorly.

• Excretion is carried out by modified excretory cells or canals, often called renette cells.

• They have complex systems of reproduction. In most instances, sexes are separate; female organs include ovaries, oviducts, and a uterus. Males have testes and copulatory spicules.

Economic Importance of Aschelminthes

While some nematodes help in decomposition and nutrient recycling, others are harmful parasites of plants, animals, and humans, causing economic loss in agriculture and livestock.

• Nematodes are deeply involved in nutrient cycling and structural maintenance of soil through decomposing organic matter and by controlling microbial populations.

• They contribute to the digestion of organic materials, thereby allowing for the re-circulation of nutrients through an ecosystem.

• Most nematodes, however, currently represent very important agricultural pests that damage crops due to root infestation and lead to economic losses.

• Parasitic nematodes present several different diseases that cause pathogens in both humans and animals, resulting in health challenges and economic costs.

• Nematodes, particularly Caenorhabditis elegans, have been used as model organisms in laboratory research due to the simplicity of these organisms and their well-mapped genetic structure.

Important Examples of Aschelminthes

Here are a few examples of Aschelminthes which you should know:

Aschelminth Diseases

Aschelminthes cause diseases such as ascariasis, filariasis, and hookworm infections in humans, which can lead to nutritional deficiencies, tissue damage, and developmental issues, especially in children. Common diseases caused by Aschelminthes are

• Ascariasis: This is an infection by the helminth Ascaris lumbricoides, which mainly causes abdominal discomfort and malnutrition.

• Hookworm infections: Infection by species such as Ancylostoma and Necator causes conditions such as anaemia and protein deficiency.

• Filariasis is caused by filarial worms and might lead to conditions such as elephantiasis.

• Enterobius vermicularis is the pinworm that causes enterobiasis. It makes it difficult to sleep and itches the posterior region.

Common Diseases Caused by Aschelminthes

Aschelminthes Quick Revision Notes

• Aschelminthes are commonly known as roundworms and are mostly represented by nematodes.

• They are triploblastic, bilaterally symmetrical, and pseudocoelomate animals.

• Their bodies are elongated, cylindrical, unsegmented, and covered by a tough cuticle.

• They exhibit an organ-system level of organisation.

• The digestive system is complete, with a distinct mouth and anus.

• Specialised respiratory and circulatory systems are absent; gas exchange occurs through diffusion.

• Excretion takes place through an excretory canal system.

• Sexes are usually separate (dioecious), and fertilisation is internal.

• Aschelminthes occur in soil, freshwater, marine habitats, and as parasites in plants and animals.

• Common examples include Ascaris, Wuchereria, Ancylostoma, and Enterobius.

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