The phylum aschelminthes are commonly called roundworms. They are called so because their body appears circular in the transverse cross-section. There are about 15000 species of roundworms. Phylum aschelminthes characteristics are that they may be free-living or parasitic. They may be aquatic or terrestrial. Rhabditis is a terrestrial aschelminth that lives freely in the soil rich in organic matter. Parasitic roundworms live in plants and animals. Ascaris, Wucheria, etc are common animal endoparasites. They have bilaterally symmetrical body symmetry. These are the key features of phylum aschelminthes.
Characteristics of Aschelminthes
Aschelminthes have the organ-system level of organization. The organs associate to form a distinct system concerned with a specific physiological function such as respiration, digestion, circulation, excretion, and reproduction. They are triploblastic animals. Ascaris and guinea worm are phylum aschelminthes examples. Aschelminthes possess the pseudocoelom and hence are called pseudocoelomates. They possess a body cavity but it is not linked with the mesoderm. In between the endoderm and ectoderm mesoderm is present. They have an elongated cylindrical body that tapers at both ends. The digestive system in aschelminthes is complete having two separate openings. The alimentary canal is a straight tube extending from the mouth at one end to the anus at another end of the body. They have a well developed muscular pharynx Their food mainly consists of blood, lymph, partially or fully digested food particles which are sucked by the rhythmic pumping action of the muscular pharynx.
Excretion in Aschelminthes
An excretory tube is present in the body of aschelminthes. It helps in the process of excretion in roundworms. This excretory tube collects the body waste from the body cavity and removes it out through the pore. The roundworms thus possess anus as well excretory pore, unlike the other lower animals that come below them. The phyla lower to them lacks digestive tract.
Sexes in Aschelminthes
It is short notes on aschelminthes sexes. Sexes are separate in aschelminthes. Separate sexes mean that males and females can be identified by external features. This condition is also known as dioecious. Females in this case are longer than males. Sperms and ova are produced by different individuals. Only sexual reproduction takes place. In aschelminthes external fertilization takes place. Ascaris and guinea worm have these characteristics. They are aschelminthes examples.
Short Notes on Aschelminthes Development
The development of the young ones takes place by intermediate larval stage whereas some develop directly. When direct development occurs, the young ones resemble the adult. The larva is not involved in direct development. There we can conclude that in roundworms both direct and indirect development takes place.
Phylum Aschelminthes Examples
It is commonly called roundworm and is a very common endoparasite in the small intestine of man. It is more common in children than in adults. Their body is covered with syncytial epidermis. They possess a cuticle that is resistant to the digestive enzymes of the host. Its body wall is made up of the following layers:
The cortex is made up of keratin. It is resistant to enzymes.
Matrix is present below the cortex. Below this, there are fibres that are made up of collagen. Below fibres, we have a basement membrane. And below the basement membrane, they have a syncytial epidermis below which only longitudinal muscles are present and circular muscles are absent.
The sexes in Ascaris are separate and they show sexual dimorphism. Females are longer than males. Males possess a curved tail, two pineal setae and cloaca. The males have 50 pairs of pre-anal papillae and 5 pairs of postanal papillae. Both male and females have terminal mouths. Their mouth is also triradiate, surrounded by three denticulate lips which are one median dorsal and one ventrolaterals. Cuticular lips are present on the mouth. Their lips inner margin has denticles and the outer margin has sensory papilla. The mid-dorsal lip bears two minute double papillae while each ventrolateral lip bears towards its ventral side a double papilla and towards the lateral sides a single papilla and a special reduced papilla known as amphid. The excretory pore is situated mid ventrally which is a little behind the mouth. There is a single huge excretory cell called renette cell or gland cell in the H shaped canal system. It consists of two longitudinal excretory canals connected anteriorly by the transverse canalicular network. The Female has a posterior transverse anus and separate gonopore situated ventrally at about ⅓ distance from the anterior end. Fertilized eggs of Ascaris pass out of the body of the host through faecal matter. Ascaris is monogenetic which means that no intermediate host is required for its development. The egg is oval in shape with three protective coverings. The coverings are the outer protein layer, middle chitinous shell and the inner membrane made of esterified glycosides. Infective eggs of Ascaris remain viable for six years. Its zygote will develop into a juvenile within 10-14 days in soil. The larva is called rhabdoid for its close resemblance to rhabditis.
2. Dracunculus Medinensis
It is commonly called guinea worm. It occurs in the subcutaneous tissue of man. The toxic secretions of the parasite cause blisters on the skin of the host. It is a digenetic parasite. The human being is its primary host and cyclops is the intermediate host. Sexes are separate. The Female is long while the male is short. The parasite causes itching, diarrhoea, nausea, and vomiting. This worm has been completely eradicated from India. These two are phylum aschelminthes examples.