Phylum Aschelminthes, Examples, Practice Problems and FAQs

We all heard about parasites. Parasites are those organisms which depend on other living organisms for their survival. These parasites may harm the life of the host organism. Have you ever felt some worms in your anal part? This will give itching or infections in that area. Yes they are the pinworms.

Just think about a worm that is living inside our body and it is penetrating through our digestive tract or feeding on our internal organs. It is very disgusting, right?!! This happens when we don't have good water to drink or when we live in an unhealthy environment. So these worms are real villains who can infect us and in serious cases this may lead to death.

GIF: Infection caused by Guinea worm

Most of these worms are commonly called nematodes which belong to the phylum Aschelminthes. Some of these parasitic worms need bodies of different hosts to complete their life cycle. In this way they are ensuring the survival of their larvae. Now let’s take a deep dive into the details of all these worms. 

Table of contents:

• Phylum Aschelminthes 
• Examples 
• Practice Problems
• FAQs 

 Phylum Aschelminthes

Aschelminthes has been derived from two words ‘nema’ meaning thread and ‘helmin’ meaning worm. Hence, the members of phylum Aschelminthes are also called nematodes. 

General characteristics of phylum Aschelminthes

The following are the major characteristics of phylum Aschelminthes:

Habitat

Aschelminthes or nematodes are also known as roundworms as their cross section is round in shape. If we consider the habitat of aschelminthes, they exist in terrestrial as well as aquatic (freshwater or marine) habitats. Several hundred billion microscopic free-living nematodes can be found in a hectare of humus-rich soil. They can be parasitic like Ascaris or free-living like Caenorhabditis elegans. 

Fig: Parasitic and free roundworms

Bilateral symmetry

They have a body with bilateral symmetry, which means the body can be divided into two equal and identical halves (right half and left half, through only one plane) called median sagittal plane. Their body has tapering ends. 

Level of organisation 

Their level of organisation can be described as the organ system level as their organs are involved to achieve a particular function from the organ system. 

Triploblastic

The germ layer organisation is triploblastic, which means the cells are arranged in three embryonic layers, outer ectoderm, inner endoderm and middle mesoderm. 

Fig: Triploblastic organisation in Aschelminthes

Outer body layers

Roundworms have an exterior cuticle, a syncytial epidermis, and a muscle layer on their body wall. A continuous layer of cytoplasm with dispersed nuclei makes up the syncytial epidermis. Only longitudinal fibres make up muscle layers. There are no round muscles.

Pseudocoelomates

Aschelminthes are pseudocoelomates. Pseudocoelomates have their body cavity present between the body wall and gut wall which is not lined by mesoderm or coelomic epithelium. Mesoderm here is present as scattered pouches between ectoderm and endoderm.

Fig: Pseudocoelom in Aschelminthes

Major systems

Nematodes do not have any lateral appendages. Their respiration occurs through the surface of the body. But the circulation is underdeveloped. They also have a nervous system with a nerve ring and nerve cord running throughout the length of their body.

Now we know about the general characters of aschelminthes. Since they are classified under the kingdom of Animalia, they will also have specific digestive and excretory systems depending on their mode of nutrition. Since they include under the lower invertebrates, it will not be much advanced, but let's check out how it works.

Alimentary canal in Aschelminthes

Roundworms possess a complete digestive system with a distinct mouth, digestive tract and anus. They have a well-developed muscular pharynx. So they have an organ system level of organisation. 

Fig: Digestive system in Aschelminthes

Excretory system in phylum Aschelminthes

The excretory system of roundworms consists of two distinct components.These include excretory tube and excretory pore. The excretory tube helps in waste removal from the body cavity. Excretory pore serves as an outlet for elimination of wastes from the body.

Fig: Excretory system in Aschelminthes

Sexual dimorphism in phylum Aschelminthes

Roundworms are dioecious in nature, i.e., have separate sexes. The male platyhelminthes are shorter than female ones, hence, sexual dimorphism is distinctly observed in them. The hind ends of males are curved ventrally. Pineal spicules are present in males which helps them in the copulation. 

Fig: Sexual dimorphism in roundworms

Mode of reproduction of Aschelminthes

They have internal fertilisation. They show cross fertilisation. 

Cross fertilisation

Roundworms being dioecious have different individuals producing male and female gametes, hence, they undergo cross fertilisation.

Internal fertilisation

It is characterised by fertilisation of gametes inside the parental body (female body).

Development of Aschelminthes

During the development of Aschelminthes the fertilised eggs are converted into larvae. This larvae will undergo moulting and gradually grow into an adult worm. If we consider the modes of development of Aschelminthes, they have direct and indirect development.

Fig: Modes of development in Aschelminthes

Direct development

In the direct development the young ones and adults look alike. Here the young one directly develops into a mature individual without undergoing metamorphosis. 

Indirect development

In the indirect development the young ones and adults do not look alike. Development from egg to adult involves intervening larval stages which are morphologically distinct from the adult forms.

Sense organs of Aschelminthes

Some of the sense organs of the nematodes are papillae, amphids and phasmids. 

Papillae 

They are raised structures that are found on the lips on the sides of the anterior end in both male and female parasites. It is also found in front and behind the cloacal aperture. The function of all papillae is touch or tactile. 

Amphids

They are chemoreceptors and are found on the lips. 

Phasmids 

They are glandulosensory unicellular glands placed on the lateral sides of the posterior end.

Examples of Aschelminthes

Ascaris

Intestinal roundworms are the most frequent name for these worms. Ascaris lumbricoides is an endoparasite that lives in man's small intestine, particularly in children. It features a pinkish cylindrical body with tapered ends on both sides. The mouth is delimited on the front by three lips: one dorsal and two ventrolateral. The lips have gustatory or taste receptors in the form of papillae and amphids. It feeds on the host's partially digested meal and breathes anaerobically. Sexual dimorphism is visible in the animal. Males are smaller than females, with curled ventral hind ends with pineal spicules to facilitate copulation. There is no intermediary host for them. They show larval stages called rhabditiform in the life cycle. Taking egg-infested food or water causes infection. In humans, it causes ascariasis.

Fig: Ascaris lumbricoides

Life cycle of Ascaris

The roundworm Ascaris lumbricoides infects people through the faecal-oral pathway. Adult females shed their eggs in the faeces. Fertilised eggs embryonate and become infectious in soil after 18 to several weeks, depending on environmental conditions. Infections occur when a person drinks or eats water contaminated with embryonated eggs.

When an egg is ingested (1), it hatches into a rhabditiform larva in the duodenum, which subsequently enters the gastrointestinal tract's wall (2). They then penetrate the bloodstream and enter the body. They then enter pulmonary circulation and break loose in the alveoli after passing via the right heart. They subsequently break out from the capillaries and invade the surrounding air spaces.

The juvenile worm then crawls from the lung (3) to the pharynx, gets coughed up, and then swallowed again, before moving through the stomach (4) to the small intestine and maturing into an adult worm. Within 60–65 days of being swallowed, they start laying eggs. Fertilisation is now possible, and the female can generate up to 200,000 eggs each day for the next 12 – 18 months. After two weeks in the soil, these fertilised eggs become infectious; they can survive in the soil for up to ten years.

Fig: Life cycle of Ascaris

Wuchereria 

Wuchereria bancrofti, also known as a filarial roundworm, is a parasite. It is long, cylindrical and thin. The adult worms are smooth with rounded ends. It is almost transparent and white in colour. The size and structure of the tail tips distinguish males and females. The male worm is smaller and has a tail that curves ventrally. Adult males and females are frequently curled together, making separation difficult. Females are ovoviviparous, that means they produce young ones by means of eggs that are hatched normally within the body of the parent. They can produce thousands of microfilariae. They infect the lymphatic system and cause lymphatic filariasis. A number of mosquito vector species distribute these filarial worms. Tropical eosinophilia, an asthmatic disease, is also a rare side effect of this worm infection.

Fig: Wuchereria bancrofti

Life cycle of Wuchereria

Wuchereria bancrofti has two hosts that it utilises to complete its life cycle. The definitive host is human, and the intermediate host is mosquito. The adult parasites normally seen living in the lymphatic system of human beings. Microfilariae, the first-stage larvae, are found in the circulatory system of humans.

The microfilariae are carried by a vector, which is usually a mosquito species from the genera Culex, Anopheles, Mansonia, and Aedes. The microfilariae mature into motile larvae termed juveniles inside the mosquito, which travel to the labium after about ten days. Wuchereria larvae are deposited from the mouthparts of the infected mosquito onto the skin of the potential host when it has its next blood meal. Then the larvae will migrate into the bloodstream of the new human host through micro cuts in the dermis or the tract created by the proboscis.

The larvae go to regional lymph nodes, which are mostly found in the legs and genital area, via the lymphatic system. Over the course of a year, the larvae mature into adult worms and reach sexual maturity in the afferent lymphatic vessels. The adult female worm can produce thousands of microfilariae after mating, which then migrate into the bloodstream. A mosquito vector can bite an infected human host, ingest the microfilariae, and thereby repeat the lifecycle. The organism does not multiply in the mosquito, which serves as its intermediate host.

Fig: Life cycle of Wuchereria bancrofti

Enterobius

The pinworm is the common name. Man's large intestine, particularly that of children, is infected by Enterobius vermicularis. The worms are whitish in colour and range in length from 3 to 10 mm. Females crawl to the anal region at night to deposit eggs, producing extreme itching. Infection occurs when eggs are consumed with food or water. Living worms frequently pass out in the faeces. Enterobiasis or oxyuriasis is a condition caused by them.

Fig: Enterobius vermicularis

Ancylostoma 

Hookworm is the common name of this parasite. It is a human endoparasite that lives in the small intestine. It feeds on blood. Its larvae enter the human body through the skin of the foot after emerging from the damp Earth. Ancylostomiasis is caused by it.

Fig: Ancylostoma duodenale

Classification of phylum Aschelminthes

Phylum Aschelminthes can be classified into two two classes. They are Aphasmidia and Phasmidia.

Aphasmidia

This class lacks phasmids or chemoreceptors. There are many types of amphids, or taste receptors in this class of nematodes. The number of excretory organs is reduced or non-existent.

Examples include Trichinella, (Trichina worm), Trichuris (Whipworm), etc.

Phasmidia

Phasmids are found near the body's hind end. Amphids have pore-like structures and are found near the anterior end. They are glandular, and their function is meant to be gustatory. The excretory organs in this class are well developed. Examples include Ascaris (Giant intestinal roundworm), Enterobius (Pinworm), Ancylostoma (Hookworm), Wuchereria (Filarial worm), etc.

Diseases caused by parasitic nematodes

Now we know that there are nematodes which live as a parasite in the human body. So Infections will be caused by these nematodes. Sometimes it may be fatal too. Now we will discuss some of the infectious nematodes and diseases caused by them.

Practice Problems

Q1. Choose the appropriate distinction between a male and a female Ascaris worm.

A. i and ii only 
B. ii and iii only 
C. ii only 
D. iii only 

Solution: Ascaris is a member of the Aschelminthes phylum. Ascaris is dioecious, meaning it has two sexes. Males and females have different morphological appearances. Females are longer than males. Males have hooked or curved posterior ends, whilst females have a straight posterior end. In males, the cloaca is the final section of the rectum. It is in the back of the body and accepts both faeces and sperm. Females do not have it. Hence the correct option is c.

Q2. Animals that are bilaterally symmetrical and triploblastic belong to which of the following phylums? 

A. Aschelminthes (roundworms) 
B. Ctenophores
C. Sponges 
D. Coelenterates (Cnidarians) 

Solution: Aschelminthes are bilaterally symmetrical triploblastic animals. Bilateral symmetry occurs when the body of an organism can be divided into two equal halves by cutting it in one specified plane running through the centre called median sagittal plane. All three germinal layers are present in triploblastic animals: ectoderm, endoderm, and mesoderm. Ctenophores and Coelenterates (Cnidarians) are diploblastic and radially symmetrical. They have only two germinal layers - outer ectoderm and inner endoderm. They can be cut in any plane going through the centre to produce equal halves. Sponges are asymmetrical, meaning they cannot be split into halves in any plane and are diploblastic. Hence the correct option is a.

Q3. What type of worm is Wuchereria bancrofti?

A. a Platyhelminthes
B. a sponge
C. the filarial worm
D. a flatworm

Solution: Wuchereria bancrofti is a member of the Aschelminthes phylum. Roundworms have a round cross-section, they belong to the Phylum Aschelminthes. It is also known as the filarial worm because it causes filariasis. Hence the correct option is c.

Q4. What are the sense organs of nematodes?
Answer: Some of the sense organs of the nematodes are papillae, amphids and phasmids. Papillae (raised structures) are found on the lips on the sides of the anterior end in both male and female. It is also found in front and behind the cloacal aperture. The function of all papillae is touch or they are tactile in function. Chemoreceptors, or amphids (pits), are also found on the lips. Phasmids are glandulosensory unicellular glands placed on the lateral sides of the posterior end.

FAQs

Q1. What is a hydroskeleton? 
Answer: A hydrostatic skeleton, also known as a hydroskeleton, is a structure that consists of the coelom, a fluid-filled chamber, and the muscles that surround it. In open circulatory systems, this fluid in the coelom (haemocoel), also known as hemolymph, is equivalent to a combination of blood and interstitial fluid. The contraction and relaxation of muscles against the haemocoel wall causes fluid pressure to be localised in a few areas of the body. The animal's body moves as a result of this activity. In addition to this function, a hydrostatic skeleton serves as a body support system and can be employed by the organism to change its shape. Aschelminthes has a hydroskeleton.

Q2. What are the types of larvae present in roundworms?
Answer: During the development of aschelminthes the fertilised eggs are converted into larvae. This larvae will undergo moulting and gradually grow into an adult worm. Three kinds of larvae found in aschelminthes are filariform larvae, microfilariae larvae and rhabditiform larvae. Filariform larva is found in Ancylostoma, microfilaria larva is found in Wuchereria and rhabditiform larva is found in Ascaris and Enterobius.

Q3. Why is Caenorhabditis elegans so significant in science?
Answer: Caenorhabditis elegans is a nematode that lives in the soil. For a growing number of researchers, it is the model organism of choice. It has been studied and researched extensively, and it is likely the best-understood animal on the planet. It is suitable for speedy and effective analysis of gene function due to its seeming simplicity, fine genetics, the presence of a full molecular toolset, and a complete genome sequence. It is the first mammal whose entire genome has been sequenced.

Q4. Which human endoparasites are viviparous?
Answer:  Trichinella spiralis is a viviparous human endoparasite. Trichinella spiralis is a nematode parasite that copulates in the colon, then the male dies and produces larvae that enter the bloodstream to reach the muscles. Vivipary is indicated by the presence of larvae.

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