LARVAL FORMS OF TREMATODA, CESTODA AND ANNELIDA

Introduction

• Helminthes constitute a large assemblage of ‘worms’ of comparatively simple and varied organization .
• Platyhelminthes includes flatworms (platy-flat , helminthe-worm) . they are bilaterally symmetrical , acoelomate , triploblastic invertebrates without an anus . the flatworms are both free living as well as parasitic.
• They are dorso ventrally flattened like a leaf. The digestive system is completely absent from cestoda and acoela .
• They are hermaphrodite i.e., both male and female reproductive organs are present in the same animal . fertilization is internal in them. Self or cross fertilization takes place in them.
• Their development is direct or indirect . Endoparasites show usually indirect development with may larval stages. Their life cycle is completed in one or two hosts.

Platyhelminthes is divided in to three classes :-

1. Tubullaria
2. Trematoda
3. Cestoda

Larval forms of class trematoda

     General characters of trematoda :-

• They are generally ecto or endoparasite.
• Body is leaf shaped or cylindrical and covered with cuticle.
• Epidermis , rhabdites and cilia (except in larval stages) are absent.
• Suckers and sometimes hooks are present.
• Digestive tract with an anterior suctorial mouth , simple pharynx and two forked many branched intestine.
• Life cycle is simple or complicated.
• They generally includes flukes.

Miracidium larva

• The first larval stage develops in 9 to 15 days under optimum temperature of 22 – 25 degree C , until it is ready to hatch . this is known as the miracidium larva
• When the larvated egg capsule comes in contact with water , the lid or operculum is dissolved and the miracidium is hatched by the capsules . it is a free swimming larva living in ponds .
• Which is conical in shape . the anterior end is broad and the posterior end is narrow.
• At the anterior end inside the body , there is a sac like gland called apical gland . it opens at the apical papilla by a duct .
• Two sac like glands are located on the sides of the apical glands. They are called penetration gland.
• A large brain is present  . The interior of the larva is filled with groups of specialized cells called as germ cells.   

             (when it comes in contact with the snail limnaer truncatula. The miracidium penetrates in to the body of the snail . it reaches the digestive gland of the snail and gets transformed in to another larva called as sporocyst).

Sporocyst larva

• Sporocyst develops from miracidium .
• It is the 2nd larva of liver fluke. It lives in digestive gland of         snail.
• The epidermal cells along with the cilia are shed primitive  gut, cephalic glands , brain, and eye spots are degenerated and larva becomes like an elongated sac.
• About 0.07 mm long covered with a thin cuticle.
• The sub-epithelial cells , muscles and mesenchyme remain as in the previous stage.
• The protonephridium of each side divides in to two flame cells but they are open to outside by a common excretory duct. Besides, the sporocyst may be called as germ balls.
• The sporocyst may be called as a living cyst moving about in the tissue of the host absorbing nourishment from it.
• This germ cells divide and redivide to form the next larva called as redia larva . each sporocyst can produce 5-8 larvae.

Redia larva

• Redia larva develops from the germ cells of sporocyst.
• It lives in the digestive glands of snail and cylindrical in shape. The body is covered by cuticle.
• The anterior end has a mouth ,behind the mouth there is a muscular ring called as collar or opening called birth pore is located.
• Near the posterior end a pair of projection is found . .They are called as lappets. They are used for locomotion.
• The mouth leads into a pharynx which ends in a sack like intestine.
• To protonephridia are located inside the body and each protonephridia formed of many flame cells.
• The cavity of redia larva is filled with germ cells . The germ cells of daughter redia develop  into the next larva called as cercaria . They come out trough birth pore.

Cercaria larva

• Each redia produces about 20 cercaria . They lives in the digestive gland of snail.
• Its body is tadpole like rounded or oval with a long simple tale. Its length about 0.25 mm to 0.35mm.
• Its body surface is covered with cuticle.
• Digestive system consist in this larva . Numerous flame cells are located inside the body .
• The cercaria lives for 3 days and it is transformed into another larva called as metacercatria.

Metacercaria larva

• The cercaria looses its tail and the  cystogenous gland secretes a cyst around larva. The encysted cercaria is called metacercaria.
• It is found attached to the grasses or near to water .
• shape- round , germ cells are present .
• In this larva tail and cystogenous gland have degenerated and a thick hard cyst wall is present .
• Flame cells have increased and the germ balls are present.
• The metacercariae are in fact the juvenile flukes. They develop further only when swallowed by sheep.
• Metacercariae survive only for a few weeks

Larval forms of class cestoda

General characters of cestoda :-

1. Cestodes are the most specialized of all the flatworms they are all endo parasites of vertebrates from fishes to mammal.
2. They are commonly called as tapeworms.
3. They are long , flat , ribbon like in shape.
4. They are depressed , headless and limbless worms with no coelome , no circulating mechanism , no skeleton , no distinct excretory system and no food canal.
5. The body contains an osmo- regulatory or water- expulsion system

Hermaphorditism is of universal occurance  among tapeworms , except in dioecoesisa ,a taenioid genus which is not only dioecious but sexually dimorphic. The gonads are arise from the mesenchyme cells.It is a digenic parasite. The egg develops in to a larva called hexacanth or onchosphere larva.

Onchosphere (hexacanth larva)

• It is the larva developed from the capsule . It remains in capsule it is spherical in shape.
• It is covered by two cover rings namely an outer shell and an inner embryonic membrane.
• The interior is filled with a mass of cells.
• The embryo has 6 pairs of hooks hence it is also called as hexacanth larva(hexa-6,canth-hook).
• The oncho sphere remains inside the gravid proglottid.
• It is passed out along with faeces further development takes place only inside the pig.

Infection to pig :-

• When a pig eats the faeces, the onchosphere inters the intestine of the pig . Where the embryonic  membrane dissolves.
• The larva penetrates the intestine wall with the help of hooks then reaches the muscles it looses the hooks and develops into the next larva called as cysticercus or bladder worm.

Cysticercus larva or bladder worm

• Cysticercus developes from the onchosphere . It has following salient features:-
• It lives in the muscles of the pig.
• it is formed of a fluid filled vesicle called bladder.
• The bladder is covered by two coverings namely an outer cuticle and an inner dermal  layer.
• The flesh of pig containing the cysticercus is called measly pork.
• Further development of cysticercus occurs inside the man.

Larval forms of annelida

General characters of annelida

1. Mostly aquatic, some terrestrial burrowing or tubicolous. Some commensol and parasitic.
2. Body elongated bilaterally symmetical , triploblastic  ,truly coelomate and metamerically sequented into similar metamerces.
3. Locomotory organs are segmentally repeated chitinous bristles called setae or chaetae embedded in skin. May be borne by lateral fleshy appendages or parapodia.
4. True coelom, schizocoelous . Mostly well developed except in leeches.
5. Ventrally divided into compartments by transverse septa coelomic fluid with cells or corpuscles.
6. Sexes are separate cleavage spiral and determinate larva when present is a trocophore .
7. Regeneration common .
8. Respiration by moist skin or gills of parapodia and head.
9. Digestive system straight and complete digestion entirely

Example :- Neries – nereis is hermaphroditic or sexes are separate cleavage spiral and determinate larva, when present is a trocophore.

Trochophore larva/ trochosphere larva 

• small translucent, free – swimming larva.
• Characteristics of marine annelids and most groups of mollusk trochophore are spherical or pear shaped and are gridled by   ring of cilia .
• The prototroach , the enables them to swim and it is a sensory plate an apical tuft of cilia and an ocellus(simple eye).
• Below the prototroach are the mouth , stomach , anus and other structure including the solenocyte.
• The function of which seems to be maintained proper internal salt water balance .
• In some molluscs (as gastropoda) and bivalves the larva develop into second stage , the veliger before metamorphing to adult form

Conclusion

• Platyhelminthes are mainly flatworms .They are called as flat worms because they are much flatted dorso ventrally. The phylum includes three classes- turbellaria or free living.
• The trematods or flukes are either external or internal parasites, the cestodes or tap worms are internal parasites.
• The termatoda class includes three order . The fasciola hepatica which is an example of order digenea have mainly 5 larval forms (i) miracidium (ii) sporocyst (ii) redia (iv) Cercaria (v) metacercaria.
• The taenia solium which is an example of cestoda completes its lifecycle into two hosts first is man and second is pig.
• Annelids have trochophore or trochosphere larva which further develops only in phylum mollusca in to veligers larva before metamorphing in to adult.