Five important larval stages that come one after another in the development of fasciola

In Fasciola development is indirect and complicated because it involves five larval stages that come one after another:

Cleavage:

Cleavage is holoblastic and unequal and starts when eggs remain in the uterus. First division of zygote results in two unequal cells, a larger somatic cell and a smaller propagatory cell.

Somatic cell divides to give rise to larval ectoderm and tissues. Propagatory daughter cell divides further into two daughter cell.

One daughter cell gives rise to remaining larval body by division and the other one forms a mass of germ cells which are clustered and stored in the posterior part of larvai body.

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Further development takes place when capsules come out from the sheep’s intestine into the water.

(1) 1^st larval stage:

Miracidium larva:

Miracidium is the first larval stage, hatches out from encapsulated embryo with the help of hatching enzyme. It is a 0.07 mm long, oval and richly ciliated, active, free swimming larva.

Its broader anterior end is mobile and non-ciliated called apical papilla or terebratorium. Its body is covered with flattened ciliated epidermal 21 plates, arranges in five rows or tiers.

Below the plates a fine layer of sub-epidermal musculature is found and it is followed by a thin layer, sub-epithelium. All three layers together form the body wall of miracidium.

Internal structure:

There is a sac like multi-nucleate mass of granular protoplasm and it is attached to the centre of apical papilla by a stalk.

A pair of large, unicellular cephalic or penetration glands opens by their narrow ends near the apical papilla.

Below the 2^nd tier of epidermal cells a brain with several associated nerve fibers is found. Above the brain, “X” shaped larval eye with two pigmented eye spots is found.

A pair of long tubular protonephridia or flame cells open to the exterior through two nephridiopores or excretory pores is found. Germ cells lie in groups and called germ balls, in the rear part of the body.

Physiology:

Miracidium does not feed swims desperately to find its suitable intermediate host (snail). Those, who find it, penetrate into it with the help of apical and penetration glands. During penetration miracidium larrva casts off its ciliated epidermis.

It soon makes its way into the digestive gland of snail, where it undergoes various changes and, in about 4 days develops into the second larval stage, the sporocyst larva.

(2) 2^nd larval stage:

Sporocyst:

It is an elongated larva of 0.7 mm length and bears the body wall same as in miracidium except ciliated epidermis. In place of it a thin cuticle is found.

Glands, brain, eye spots and apical papilla of miracidium larva degenerate and disappear in sporocyst.

Protonehridia of each side divides into two flame cells which open outside by a common excretory pore.

Sporocyst moves in the digestive gland and feeds upon tissues of the host. Its germ balls develop into the 3^rd larval stage, the redia. Each sporocyst produces 5 to 8 rediae.

(3) 3^rd larval stage:

Redia:

By rupturing the body wall of sporocyst rediae come out in the host’s tissue. It has a 1.3 to 1.6 mm long, elongated and cylindrical body.

It bears a mouth at the anterior end, a ring of muscular swelling or collar, a permanent birth pore a little behind collar, and a pair of lappets procruscula ventrolaterally near the posterior end.

Body wall is same as in sporocyst. viz., cuticle, musculature and subepithelium. Mouth leads into a short muscular pharynx, followed by an elongated sac-like intestine, Enteron or gut is lined by a single layer of cells.

Numerous unicellular pharyngeal glands open into pharynx. Protonehridia divide further and form a much branched system. Body of larva is packed with germ balls and mesenchyme cells.

Through muscular contraction aided by collar and lappete rediae larvae wander in the host’s tissue and feed. It moves into various organs but prefes to migrate into digestive glands.

During summer months germ ball of redia gives rise to a second generation of rediae. During winters germ balls of second generation rediae produce cercaria larva.

(4) 4^th larval stage:

Cercaria larva:

14 to 20 cercari emerge out from each redia through birth pour into the host’s tissue. It bears a close resemblance to the adult fluke. It is 0.25 to 0.35 mm long with an oval body and a tail for swimming.

Layers of body wall are same viz., cuticle, musculature and subeithelium, but cuticle bears backwardly directed spines.

Beneath the body wall lie several cystogenous gland cells, which secrete cyst for the next larval stage.

Mouth leads into a muscular pharynx followed by esophagus and intestine. Intestine is forking in front of the ventral sucker to form two tubular limbs.

Flame cells are found in large number, along the lateral zones, open into a pair of excretory tubules, which unite to form an excretory vesicle or bladder.

From the bladder a secretory duct arises and extends into the tail region where it bifurcates and opens out through a pair of nephridiopores. Germ cells are present.

Mature cercaria makes its way through pulmonary sac of the snail and finally escapes into the water.

The development of fluke in the snails’s body from the stage of miracidium larva to cercaria larva takes 35 to 65 days depending upon availability of food and temperature.

After an active life of two or three days it settle down on the leaf of aquatic plants and undergoes encystment to become metacercaria.

(5) 5^th larval stage:

Metacercaria:

Metacercaria is rounded, 0.2 mm in diameter, surrounded by a thick and hard cyst. It contains a large number of flame cells, but its excretory bladder opens out directly through single pore.

Tail and cystgenous glands become degenerate. Metacercaria is in fact the juvenile fluke, also called marita. Germ cells or genital rudiments are present as such.