In this article we will discuss about Polystoma Integerrimum:- 1. Habit and Habitat of Polystoma 2. Structure of Polystoma 3. Body Wall 4. Alimentary Canal 5. Excretory System 6. Nervous System 7. Reproductive System 8. Life Cycle.
Contents:
- Habit and Habitat of of Polystoma Integerrimum
- Structure of of Polystoma Integerrimum
- Body Wall of Polystoma Integerrimum
- Alimentary Canal of Polystoma Integerrimum
- Excretory System in Polystoma Integerrimum
- Nervous System of Polystoma Integerrimum
- Reproductive System in Polystoma Integerrimum
- Life Cycle of Polystoma Integerrimum
1. Habit and Habitat of Polystoma Integerrimum:
Polystoma integerrimum is an endoparasite in the urinary bladder of frogs and toads. Heavy infection is, however, not known in the adult host. Young forms may be found attached to the gills of tadpoles.
2. Structure of Polystoma Integerrimum:
The body of Polystoma integerrimum is leaf-like and dorso-ventrally flattened and measure not more than 3 cm in length. It is attached to its host by opisthaptor. The opisthaptor contains three pairs of suckers, two anchors and six marginal chitinous hooks. At the anterior end is the mouth and shortly behind on the ventral side is the gonopore.
Head glands and head organs are commonly present. The secretions of these glands and the adhesive nature of the organs suggest that these function primarily as auxiliary attachment organs. Near the suckers, certain gland cells are frequently present in the body wall and these cells secrete into prohaptors and opisthaptors.
3. Body Wall of Polystoma Integerrimum:
The body surface of Polystoma integerrimum is covered by a thin layer of non-cellular cuticle. Immediately beneath the protective cuticle is a thin layer of circular muscles. Below the circular muscles is found a thin layer of diagonal muscles and below the diagonal muscles is a thick layer of well- developed longitudinal muscles.
The region between the body wall and the internal organs is packed with a loose parenchyma consisting of cells, fibrils and spaces. If the inner and outer parenchyma is separated, the medial zone is termed the medullary parenchyma and the outer zone the cortical parenchyma. The parenchyma serves as a site of glycogen storage.
4. Alimentary Canal of Polystoma Integerrimum:
The mouth is located at the anterior end of the body, flanked on each side by a prophator. The mouth leads into the muscular pharynx which in turn leads into the oesophagus. The oesophagus opens into the intestine. The intestinal tract is in the form of an inverted Y with the caeca bifurcating from the posterior extremity of the oesophagus which represents the stem of Y.
The caeca not only give rise to numerous diverticula, but some of these actually extend into the disc-shaped opisthaptor. The intestinal caeca are lined with epithelial cells that are either closely packed or sparsely arranged.
Polystoma integerrimum feeds primarily on blood, sloughed epithelial cells and mucus. But it can also derive oxygen and other chemicals from the aquatic environment in which it is bathed. Hence, the body wall is important as an absorptive layer. Although nothing is known about the digestive enzymes present, a black pigment is often produced that is a breakdown product of ingested haemoglobin.
5. Excretory System of Polystoma Integerrimum:
The excretory system in Polystoma integerrimum is protonephritic type with flame cells at the end of collecting tubules. There are two main lateral tubes which begin anteriorly and extend posteriorly. Each tube makes a U curve prior to reaching the posterior end of the body and then extends anteriorly.
Toward the end of each ascending tube, there is a swelling known as contractile bladder. The tubes leaving the bladder empty to the outside through two separate nephridiopores situated dorso-lateral to the mouth. The flame cells are located at the free ends of branches of these main collecting tubes.
6. Nervous System of Polystoma Integerrimum:
In Polystoma Integerrimum, the brain is arranged in a well formed circumoesophageal ring. Nerve fibres extend from circumoesophageal ring anteriorly, laterally and posteriorly, one pair being dorsal, one pair ventro-lateral, and one pair ventral.
The ventral nerves, which are most highly developed, are often connected by a series of transverse commissures. Branches of nerve fibres innervate the various sucker muscles and other portions of body. One or two pairs of eye spots are commonly present. Each eye is composed of a rounded retinal cell surrounded by rods made up of pigment granules.
7. Reproductive System of Polystoma Integerrimum:
The male reproductive system contains a single testis which lies in the middle of the body. Vas deferens passes forward and terminates at the tip of the penis through which it traverses. Penis opens to the outside through the genital atrium located on the ventral surface of the body behind the caecal bifurcation.
The female reproductive system contains a single ovary situated towards the anterior side. The ovary is elongated. The oviduct arises from the surface of the ovary. Two longitudinal vitelline ducts are connected by a transverse duct; a median vitelline duct connects with the oviduct and another genito-intestinal canal opens into the vitelline ducts, one on each side, are a pair of vaginae.
Oviduct after receiving the vitelline duct continues as ovo-vitelline duct and opens into a small chamber called ootype where eggs are assembled.
Numerous unicellular glands collectively known as Mehlis’ gland, surround and secrete into the ootype. The secretions of these glands apparently serve as a lubricant that facilitates the passage of completely formed eggs from the ootype up the uterus. A uterus containing fertilised eggs comes out of the ootype to open into the genital atrium.
Functionally, the common vitelline duct is the tube through which the shell-forming materials and some yolk are carried into the oviduct. The seminal receptacle serves as a storage for spermatozoa received by the female during copulation. The male cirrus is inserted in the vagina of the female during copulation and spermatozoa are introduced down this tubular canal.
8. Life Cycle of Polystoma Integerrimum:
The monoecious adult of Polystoma integerrimum inhabits the urinary bladder of frogs and toads. During the winter months the gonads are non-functional, but activity commences with the coming of spring, producing large eggs. The number of eggs produced ranges from 4 to 122 per day for one week. These eggs are expelled to the exterior.
Embryonic development within the egg-capsule (shell) is affected by temperature. At suitable temperature above 50° F., development of the onchomiracidium normally takes less than three weeks. If, however, the temperature drops below 50° F., development may take six to thirteen weeks.
The correlation between the hatching of P. integerrimum eggs and the development and metamorphosis of the frog is one of astounding natural synchronisation and suggests a hormonal influence.
The barrel-shaped onchomiracidium, which bears 16 arrow-shaped hook-lets on its opisthaptor, emerges from the egg and becomes free-swimming at the time that the tadpoles lose their external gills and acquire internal ones.
The larva actively seeks out such a tadpole and enters the gill-chamber, in which it becomes attached to the gill-filaments by its armed opisthaptor. In this attached position, development continues for about eight weeks while the larva subsists on mucus and sloughed host cells.
When the frog undergoes further metamorphosis by losing its gills and developing into a young adult, the worm passes out of the branchial chamber, migrates down the host’s alimentary canal, and eventually becomes established in the host’s urinary bladder, which by this time has developed.
During its migration, the larva loses its ciliated epidermis through atrophy, develops six suckerlets on the cotylophore, loses its larval hooks, and develops adult-type anchors, in other words, the larva matures. In the bladder of the frog, sexual maturity of the parasite is attained within three years.
In exceptional situations in which larva of Polystoma integerrimum becomes attached to the external gills of a younger tadpole, an unnatural acceleration in larval development takes place.
Shortly before the tadpole metamorphosis into an adult, the Polystoma larva develops into a neotenic form, i.e., it becomes sexually mature and produces viable eggs. The other anatomical characteristics of the neotenic worm are not like those of urinary bladder form.
The correlation between the maturation process of the host and the developmental pattern of the parasite again strongly suggests that the parasite is controlled by the hormonal influence of the host.
Hyman (1951) suggested that the neotenic form may be an alternating one with the urinary bladder form, whereby the larvae produced from eggs laid by the branchial form directly invade the urinary bladder of the frog through the anus. Gallien (1935), however, proposed that the larvae of branchial forms leave the host and seek out other tadpoles at the internal gill stage and follow the normal pattern after that.
Recent experiments by Miretski (1951) have revealed that maturation of Polystoma integerrimum is controlled directly or indirectly by the hormonal activity of the frog. This investigator reported that when hypophysis extract is injected into an infected immature frog, the polystomes within the frog mature within 4 to 8 days and produce a small number of eggs for approximately one week.
This period of time corresponds approximately to the time frogs spend in spawning. This synchronised mechanism results in the release of the parasite’s eggs only when the frogs enter water to breed. In addition, it also assures that by the time the onchomiracidia hatch, there are abundant tadpoles available for reinfection.
How the hypophysis extract affects the maturation of polystomes is still uncertain. It is possible that either the increased level of gonadotrophins or sex hormones, brought about by the hypophysis extract, could be responsible.