Taenia Solium – The Tape Worm


EXCRETORY SYSTEM

Excretory organs are flame cells scattered all over the mesenchyme. A flame cell is of irregular shape with granular cytoplasm and a nucleus. Bundle of cilia or flame arises from basal granules near nucleus. The excretory products pass out through excretory canals and out of excretory pores.

There are two lateral longitudinal excretory canals of which one is dorsal and the other ventral and they get connected by a nephridial plexus in scolex. Dorsal canals are confined to the anterior region of body. Ventral canals are large and extend along the entire length of body. Ventral canals of each side are connected by a transverse canal at the posterior part of each proglottid and open on the posterior end of proglottid by excretory pores. When the last proglottid is shed the terminals of the two ventral canals function as excretory pores.

REPRODUCTIVE SYSTEM

Mature proglottids are hermaphrodite having complete sets of male and female reproductive organs.

Male Reproductive Organs

Male reproductive organs consist of testes, vasa efferentia, vas deferens, cirrus or penis and cirrus sac. Testes are numerous small spherical bodies found scattered in the mesenchyme inside the proglottid. Vasa efferentia are minute capillaries that originate from testes and connect to vas deferens. The vasa efferentia from surrounding testes unite to form a common sperm duct, vas deferens, approximately in the middle of the proglottid and opens at the base of cirrus organ.

The cirrus opens into a cup shaped genital atrium, situated in the middle of the lateral margin. The genital atrium opens by the common gonopore on a genital papilla.

Female Reproductive Organs

Ovary, also called germarium, is a bilobed organ situated ventrally in the posterior part of proglottid. Both the lobes are connected by a transverse ovarian bridge. Oviduct arises from the middle of ovarian bridge and opens into the ootype, which is spherical bulb-like structure situated at the junction of oviduct, uterus and vitelline duct. The seminal receptacle stores the sperms temporarily.

Uterus is a blind sac-like structure originating from the ootype and extending towards the anterior side of the proglottid. In gravid proglottids uterus becomes large and highly branched.

Vitelline gland is situated on the posterior margin of the proglottid and connected to the ootype. The unicellullar Mehli’s glands surround the ootype and secrete a slimy substance that lubricates the eggs in uterus.

LIFE CYCLE OF TAENIA

 Normally self-fertilization occurs by the insertion of the cirrus of one proglottid into the vagina of the same proglottid and sperms are transferred. Cross-fertilization between different proglottids of the same tapeworm is also common. The sperms are stored in seminal receptacle, from where they fertilize the eggs in oviduct. After fertilization eggs are packed in the uterus and they are called oncospheres.

This 6-hooked embryo in the egg is called hexacanth, which possesses a pair of penetration glands and is surrounded by two hexacanth membranes. The gravid proglottids which pass out from the host body contain large number of oncospheres.

The oncospheres are eaten by pigs along with human faeces. In the stomach of the pig shell is dissolved but the hexacanth membranes dissolve in intestine. Hexacanth now bores through the intestinal wall with the help of unicellular penetration glands, entering the blood stream and reaches striated muscles of the pig and become encysted in a round covering to become cysticerci or bladder worms. The pork containing these cysticerci appears white spotted and is called measly pork.

Cysticercus is a bladder-like sac filled with a clear watery fluid having mostly blood plasma of the host. The wall of the bladder consists of an outer cuticle and inner mesenchyme. A thickening arises gradually on one side of the bladder, which invaginates and develops suckers on its inner surface and hooks at its bottom, forming proscolex.

When man eats improperly cooked pork, the cysticerci become active in the intestine. Proscolex turns inside out, so that suckers and rostellum come to lie on the outer surface as in the adult. The scolex anchors to the mucous membrane of the intestine and the neck produces a series of proglottids to form the strobila. It takes ten to twelve weeks for the young worm to become an adult.

PARASITIC ADAPTATIONS

The tapeworm exhibits a number of adaptive features to live in the intestine of the human beings.

  • It has suckers and hooks to anchor on the intestinal wall of the host.
  • Its body is covered externally by tegument, which protects it from host’s digestive juice.
  • Alimentary canal is totally absent as the parasite absorbs digested food of the host through its body surface. Microvilli of tegument also increase the absorptive surface.
  • The anaerobic respiration enables it to live in an environment which is oxygen-free.
  • The long flattened body provides larger surface area for its saprozoic mode of nutrition.
  • The sense organs are absent due to its sheltered place in intestine.
  • Enormous power of reproduction enables it to ensure transfer of at least a few embryos to the pig and some larvae from pig to man to maintain the continuity of the race.
  • The simplicity of life cycle lowers the hazards it has to face in transfer from man to pig and pig to man.
  • Internal osmotic pressure is higher than that of the surrounding host’s fluid or tissue, and pH tolerance range is high, 4-11.
  • Resistant shell around zygotes and embryos provides protection from unfavourable conditions.
  • Being hermaphrodite ensures self-fertilization within the same proglottid.

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