In this article we will discuss about:- 1. Habitat and External Feature of Ascaris 2. Body Wall and Cavity of Ascaris 3. Alimentary System 4. Excretory System 5. Nervous System 6. Reproductive System 7. Life-History.
Contents:
- Habitat and External Feature of Ascaris
- Body Wall and Cavity of Ascaris
- Alimentary System of Ascaris
- Excretory System of Ascaris
- Nervous System of Ascaris
- Reproductive System in Ascaris
- Life-History of Ascaris
1. Habitat and External Feature of Ascaris:
It resides as an endoparasite in the small intestine of man. The worm may migrate into other neighbouring channels. The parasites are most frequent in tropical and subtropical countries.
It is milk-white in colour but presents a reddish-yellow shade when alive. Female worms are larger than the males and are further distinguished by the presence of separate and independent genital aperture situated on the ventral surface at about one-third of the body length from the anterior (Fig. 69F).
Sexes can also be distinguished externally by the shape of the post-anal portion of the body. The post-anal portion in the male is sharply curved downwards while in the female it is nearly straight. Females are usually 20-25 cm in length and 5 mm in diameter while the males are 15-17 cm in length and 3 mm in diameter.
The body is cylindrical and tapering at both ends. The mouth aperture is anterior and terminal in position and is bounded by three lips, one median and dorsal and two ventro-lateral in position. A little up on the ventral side of the posterior end is situated the anus which is transverse in position and in males it serves also as the reproductive aperture.
In males a pair of needle-like chitinoid bodies project from this aperture which are called penial setae. A little down the anterior tip and on the ventral surface is situated an excretory pore. The female genital aperture is situated on the ventral surface at about one-third of the body length from the anterior end.
There is no independent male genital aperture. The body is marked with four longitudinal streaks or lines running along the entire length of the body. Of these four streaks, one is dorsal, one is ventral, and two are lateral in position. The dorsal and ventral streaks are narrow and pure white in living condition while the lateral ones are thick and brown in colour.
2. Body Wall and Cavity of Ascaris:
The outer surface of the body is furnished by cuticle which is thin, transparent, delicate, and wrinkled transversely (Fig. 69G). The cuticle is secreted by the ectoderm. The ectoderm underneath the cuticle forms a syncytial protoplasmic layer. Below the ectoderm is a single layer of longitudinal muscles. The individual cells of the longitudinal muscles are very peculiar in appearance (Fig. 69H).
Each cell is spindle-shaped, straightened longitudinally and bears at the middle a bladder-like mass of protoplasm containing the nucleus which faces the side of the body. This peculiar appearance is due to the differentiation of the cell into a contractile and a nuclear part.
The longitudinal muscle layer is not continuous and is arranged into four longitudinal bands, two dorso-lateral and two ventro-lateral. This is due to the fact that the ectoderm projects inwards at the dorsal, ventral and two lateral sides between the muscles to give rise to the streaks on the outer surface of the body.
The body cavity is not a coelom in the true sense as it is not lined by cell layers derived from mesoderm. The body cavity is lined externally by fibrous processes of the longitudinal muscle cells and internally by cuticle encasing the intestine. Fig. A and B depicts the body wall and disposition of major organs as seen in transverse sections.
3. Alimentary System of Ascaris:
The mouth is anterior and terminal in position and leads into a straight tube which runs along the entire length of the body (Fig. 69I.A). The mouth is guarded by three lips. Behind the mouth is buccal cavity, which leads into pharynx.
The pharynx is dilated and its wall is muscular. The pharynx draws food from the intestinal contents of the host. It leads to the intestine. The posterior part of the intestine narrows down to form the rectum which opens to the outside through anus. The entire alimentary canal is made up of single epithelial layer covered internally and externally by cuticle derived from ectoderm. Digestive glands of any kind are absent.
4. Excretory System of Ascaris:
The excretory system consists of two longitudinal excretory canals one through each lateral line (Fig. 69J). The two canals unite with each other at the anterior end and open to the outside through the single excretory aperture situated on the ventral side.
Four or six big tufts of cells with ramifying protoplasmic processes remain in close contact with the canals. It is believed that these cells collect, store and pass on the waste matter in dissolved state to the excretory canal. The whole system is neither a ciliated one nor there is any flame cell. It is presumed that the canal is an intracellular tubular extension of a single enormously elongated cell.
5. Nervous System of Ascaris:
The nervous system consists of a ring of nervous matter round the pharynx (Fig. 69B). The ring is swollen at the ventral side and is ganglion-like. The ring gives off six nerves to the anterior and six nerves to the posterior side. Of the posterior nerves, two are of considerable thickness and run along the dorsal and ventral lines up to the posterior end of the body.
The dorsal and ventral longitudinal nerves are connected with each other by transverse commissures. The tip of the ventral nerve cord swells and forms a ganglion just in front of the anus. The only sense organs are the sensory papillae situated as small elevations on the lips.
6. Reproductive System in Ascaris:
The sexes are separate [Fig. 69J(A)] and there exists a considerable degree of sexual dimorphism.
Male Reproductive Organs:
Male reproductive organs consist of a single thread-like, much-coiled structure occupying some portion of the body cavity. This structure is differentiated at the anterior region as the testis, the middle region as the vas deferens and the posterior region as the seminal vesicle.
The demarcations between these three parts are very poor. The seminal vesicle continues as the ejaculatory duct and opens into the anus. The opening is associated with a pair of chitinoid spicules called penial setae, each of which is provided with a muscular penial sac. The anus, in case of male Ascaris, is used for the elimination of faeces as well as of sperms and in some way it resembles the cloaca of toad.
The sperms of Ascaris are very peculiar as they show amoeboid movement inside the gonoduct of the female. A ripe sperm is cone- shaped in appearance having a broad base and an apex [Fig. 69J (C)]. The apex contains the acrosome and the broad base contains nucleus and mitochondria. The sperms remain non-motile in the male gonoduct.
Female Reproductive Organs:
Female reproductive organs consist of a pair of much coiled thread-like ovaries which pass into a uterus. The two uteri unite and form a muscular vagina which opens to the outside by the female genital aperture situated on the ventral surface at about one-third of the body length from the anterior end [Fig. 69J(B)].
7. Life-History of Ascaris:
Eggs are produced in huge numbers (20,000 a day) and are fertilized in the upper part of the uterus while the parasites remain in the intestine of the host. After fertilization each egg becomes enclosed in a chitinous egg shell [Fig. 69J(D)].
Shelled eggs without being segmented come out of the body of the host along with the faeces. Formation of embryo becomes complete in about two weeks. Infection is direct, i.e. intermediate hosts are not involved.
Infection results from ingesting fully developed embryos encased in shell along with food or drink. The embryos are not hatched in the stomach and pass on to the intestine where they hatch within two hours. Hatching is probably due to mechanical injury of the shell and is not comparable to a biological hatching.
A newly-hatched larva burrows the intestine and enters the lymphatic ducts or veinules. From the lymphatic ducts it is carried to the mesenteries, lymph nodes and from there to the portal vein through circulation. From the portal vein the larva comes to the liver through hepatic veins. Here they pass from interlobular to intra-lobular veins and are carried to the right side of the heart from which they are carried to the lungs.
Development and moulting take place in the alveoli of lungs and then they come down to intestine through trachea and oesophagus. The journey takes about ten days and adulthood is reached in about two months time.