Zoology Practical File PDF

# Chapter 8: Study of Parasites ## Phylum-Protozoa ### 1. Trypanosoma * **Phylum:** Protozoa * **Class:** Mastigophora * **Subphylum:** Plasmodroma * **Subclass:** Zoomastigophora * **Order:** Protomanadina * **Genus:** Trypanosoma * **Species:** gambiense **Habit and Habitat** Trypanosoma is a...

# Chapter 8: Study of Parasites ## Phylum-Protozoa ### 1. Trypanosoma * **Phylum:** Protozoa * **Class:** Mastigophora * **Subphylum:** Plasmodroma * **Subclass:** Zoomastigophora * **Order:** Protomanadina * **Genus:** Trypanosoma * **Species:** gambiense **Habit and Habitat** Trypanosoma is an endoparasite that resides in blood and the cerebrospinal fluid of humans. The common species are *Trypanosoma gambiense* and *Trypanosoma rhodeniensis*. It produces sleeping sickness and is widely distributed in South Africa. **Comments** 1. The adult form is an acellular, microscopic, fusiform elongated structure with both ends pointed. 2. The body is covered by a pellicle, which provides a definite shape to the animalcule. 3. The protoplasm is differentiated into: * **Ectoplasm:** Outer layer below the pellicle. * **Endoplasm:** Inner mass. 4. In the center of the body is a large vesicular nucleus that is differentiated into a dense central and a less dense peripheral clear part. 5. Towards the posterior end, a spherical, rod-shaped parabasal body or kinetoplast is present. 6. Near the parabasal body, a blepheroplast is also found. 7. Kinetoplast and blepheroplast are connected by a delicate rhizoplast. 8. The flagellum, which is strong, arises from the basal granule at the posterior end and extends forward towards the anterior end along the outer margin of the body. 9. It is connected with the undulating membrane, which is thin lateral, irregular, fin-like and formed by pellicle. 10. Volutin granules are scattered in the cytoplasm. 11. Contractile vacuoles and food vacuoles are absent. ### 2. Leishmania * **Phylum:** Protozoa * **Class:** Mastigophora * **Subclass:** Zoomastigophora * **Order:** Protomanadina * **Genus:** Leishmania **Habit and Habitat** *Leishmania* is an intracellular parasite of humans and other mammals. It causes leishmaniasis. *L. donovani* causes Kala-azar in humans. It lives in the endothelial cells of blood vessels and also enters cells of the liver, spleen, and bone marrow. **Comments** 1. _Leishmania_ is almost rounded with a central nucleus. 2. The blepheroplast lies anterior to the nucleus and is connected with the nucleus. 3. The flagellum is absent, but a thin filament called rhizoplast is present. 4. Sandfly, *Phlebotomus sp.*, acts as a vector and intermediate host for *L. donovani*. 5. In the alimentary canal of the sandfly, it occurs in the leptomonad stage. 6. Its anterior end is broad and posterior narrow end pointed. It has a long flagellum. 7. *L. tropica* causes oriental sore in humans spread by sandfly and bed bugs. ### 3. Entamoeba gingivalis * **Phylum:** Protozoa * **Class:** Rhizopoda * **Order:** Lobosa * **Genus:** Entamoeba * **Species:** Gingivalis **Habit and Habitat** It is commonly known as mouth amoeba. It lives in the cavities of teeth, in the tartar and debris accumulated around the roots of teeth. It may also live in abscesses of gums and tonsils. **Comments** 1. The trophozoite of *E. gingivalis* is about 12-20 μm in diameter. 2. The pseudopodia are many and with broad and rounded ends. 3. The hyaline ectoplasm is crystal clear, while the endoplasm is vacuolated. 4. It contains a vesicular nucleus, a number of leukocytes and bacteria in food vacuoles. 5. *E. gingivalis* does not form cysts. It is transmitted from mouth to mouth of persons either by contact during kissing, or while eating or drinking, or use of a contaminated toothbrush. 6. *Entamoeba gingivalis* aggravates pyorrhoea by destroying the gum tissue. ### 4. Nyctotherus * **Phylum:** Protozoa * **Class:** Ciliata * **Subclass:** Euciliata * **Order:** Spirotricha * **Genus:** Nyctotherus **Habit and Habitat** *Nyctotherus* is found as a parasite in the rectum of frogs, and in the intestine of apple snails and cockroaches. It is found associated with other parasitic protozoans like Opalina and Balantidium. **Comments** 1. The body is kidney-shaped or bean-shaped, dorso-ventrally compressed, and measures 60-120 microns in length. 2. The body surface is uniformly covered with equal-sized cilia. 3. The protoplasm contains a large macronucleus and a small micronucleus, two contractile vacuoles, and numerous food vacuoles. 4. Peristome leads into a curved cytopharynx, which ends in the protoplasm. 5. Permanent cytopyge is present on the posterior end to digest and undigested matter. 6. Reproduction by binary fission and conjugation. ### 5. Balantidium * **Phylum:** Protozoa * **Class:** Ciliata * **Subclass:** Euciliata * **Order:** Spirotricha * **Genus:** Balantidium **Habit and Habitat** *Balantidium* is a common intestinal parasite of vertebrates found chiefly in amphibians and mammals. **Comments** 1. It is oval or pear-shaped, with the anterior end slightly narrow and posterior rounded. 2. The entire body surface is ensheathed with spiral rows of uniform cilia. ### 6. Giardia * **Phylum:** Protozoa * **Class:** Mastigophora * **Subclass:** Zoomastigophora * **Order:** Polymastigina * **Genus:** Giardia **Habit and Habitat** _Giardia_ is found as a parasite in the digestive tract of vertebrates. _G. intestinalis_ is a parasite in the intestine of humans. **Comments** 1. The body is bilaterally symmetrical and pear-shaped in appearance, with the anterior end rounded and posterior end tapering. 2. The dorsal surface is convex and the ventral surface is flattened, provided with a concave sucker. 3. Axostyles from the median longitudinal axis of the body. 4. Four pairs of flagella arise from four pairs of basal granules. 5. The protoplasm contains two nuclei, two axostyles, and two parabasal bodies. 6. Reproduction takes place by longitudinal binary fission and cyst formation. ### 7. Entamoeba histolytica * **Phylum:** Protozoa * **Subphylum:** Plasmodoma * **Class:** Rizopoda * **Order:** Lobasa * **Genus:** Entamoeba * **Species:** histolytica **Comments** 1. *Entamoeba histolytica* was discovered by Lamble, and its pathogenic effects were described by Losch in 1875. 2. *Entamoeba* is a monogenetic endoparasite found in the large intestine and causes amoebic dysentery (amoebiasis). It may infect the lungs, liver, and brain. 3. *Entamoeba* secretes the histolysin to dissolve the tissue of the intestinal wall. 4. Single pseudopodium but a contractile vacuole is absent. 5. It is dimorphic, having *magna* (trophozoite), a pathogenic stage and *minuta*, a non-pathogenic form. 6. Infection is transmitted from the old host to a new host, i.e. humans by a quadrinucleated cystic form (tetranucleated cyst) through contaminated food and water or edibles. (Immature cysts are binucleated). 7. Two chromatoid bodies are present in the cyst, which are made up of RNA and supply nutrition to a cyst. Reserve food is glycogen. 8. Drugs for amoebic dysentery are Flagyl, metrogyl, emetin, and arsenic compounds. ### 8. Plasmodium Vivax * **Phylum:** Protozoa * **Class:** Sporozoa * **Subphylum:** Hemosporidia * **Genus:** Plasmodium * **Species:** vivax **Comments** 1. *Plasmodium* is commonly known as "malaria parasite". Its initial infection in humans takes place by sporozoites, which developed in mosquitos during sporogony. 2. *Plasmodium* was discovered by Charles Laveran. 3. It is digenetic, i.e. the life cycle is completed on two hosts: * humans * mosquitos (*Anopheles culicifacies*) 4. Monkeys serve reservoir hosts, and *Anopheles* as vectors. 5. Pre-Erythro cycle, Exo-Erythro cycle takes place in liver cells, and resultant forms are cryptozoites and metacryptozoites. 6. Erythrocytic schizogony takes place in RBCs of blood and is also known as Golgi cycle. Resultant forms are merozoites, also known as Schizozoites. (No. 12 to 24 in *vivax*). Merozoites secrete hemoloysin to destroy RBCs. (Lysolecithem is secreted by the spleen). 7. Haemozoin is a toxic substance formed in RBCs and is responsible for malaria fever when liberated in blood at the synchronous stage of schizont bursting. 8. The period from the entry of sporozoites (initial infection) till the symptoms of disease is known as the incubation period (8-12 days in *vivax*). 9. Gametocytes of *plasmodium* are formed in RBCs of human blood. 10. Gametes are formed in the lumen of the stomach of *Mosquito*. 11. Microgametes (gametes) are produced by exflagellation. 12. Zygote of *plasmodium* is elongated and motile and is known as ookinete or vermicule. 13. Oocyst is formed in the wall of the stomach, which is also known as sporont. These were first observed by Sir Ronald Ross. So the sexual cycle in mosquitos is also known as the Ross cycle. 14. Due to the sexual cycle phase (Ross cycle) in mosquitos, *Anopheles* is now considered as the primary host. ## Phylum-Aschelminthes ### 1. Ascaris * **Phylum:** Aschelminthes * **Class:** Nematoda * **Order:** Ascaroidea * **Genus:** Ascaris * **Species:** lumbricoides **Habit and Habitat** *Ascaris lumbricoides* is a common parasite found in the large intestine of humans. It is worldwide in distribution. The number of worms may be 500 or more in a single host. **Comments** 1. *Ascaris lumbricoides* is commonly known as the roundworm. 2. It is an endoparasite of the intestine of humans. 3. It is found in the lumen and produces anti-peristalsis to remain in the same position. 4. It produces antienzymes that destroy protein digestion in the host. 5. It is unsegmented, pseudocoelomate, and bilaterally symmetrical. 6. It is unisexual (dioecious). 7. The male is smaller with: * a curved pointed tail * two pineal setae * a cloacal opening for rectum and ductus ejaculatus 8. The female is larger with: * a blunt straight tail * a gonopore in the anterior 1/3 part of body * a separate anus for the rectum 9. The body is covered by a thick cuticle, which is made up of lipoprotein. 10. The ectoderm is syncytial (multinucleated). 11. Longitudinal lines are present, which are four in number: * dorsal - one * ventral - one * laterals - two 12. The lines are modifications of the ectoderms. 13. Longitudinal muscles are present in the body wall. 14. The number of muscle cells is 125-150 in each quadrant, i.e. 500-600 in total. 15. The mouth is surrounded by three lips that are denticulate on the inner side. 16. Sensory papillae are present on the lips. These are: * amphids - chemoreceptor/olfactory * simple papillae - tactile * double papillae - gustatory and tactile 17. Fertilization is internal and cross-type. 18. Development takes place outside the body of the host in moist soil. 19. Cleavage starts in the shelled egg. 20. The first stage juvenile is formed in 10-14 days. 21. The first moulting takes place within the shelled egg. 22. The second and third moulting takes place in the lung. 23. The fourth and final moulting takes place in the intestine. 24. Hatching takes place in the intestine of humans. 25. The larva of *Ascaris* is known as the rhabditiform larva. 26. The infective stage is a shelled egg with a second stage juvenile, i.e. an embryonated egg. 27. The larval stage is more injurious to humans, as it can reach vital organs through blood circulation, e.g. brain, and may cause serious injury. 28. The life cycle is monogenetic. 29. The larva completes a 10-day tour (or 3 weeks) after hatching in the intestine. 30. The larva passes through the liver, heart, lung, and finally, reaches the intestine again. ### 2. Wuchereria Bancrofti (filaria worm) * **Phylum:** Aschelminthes * **Order:** Filarioidea * **Genus:** Wuchereria * **Species:** bancrofti **Habit and Habitat** *Wuchereria bancrofti* lives in the lymphatic vessels and lymph nodes of humans. The life cycle is completed in two hosts, so it is a digenetic nematode. It is found in tropical and subtropical countries. **Comments** 1. *Wuchereria bancrofti* is commonly known as the human filarial worm. 2. It is mostly found in tropical regions. 3. *Wuchereria* is a dimorphic intercellular endoparasite of humans. 4. *Wuchereria bancrofti* is also known as *Filaria bancrofti* and causes elephantitis disease (Filariasis). 5. *Wuchereria* lives in lymph vessels and lymph glands of humans. 6. In filariasis, lymph vessels and glands become inflamed and obstructed, so tissue enlarges due to accumulation of lymphatic fluid. 7. Infection of *Wuchereria* takes place through *Culex* mosquitos. 8. The life cycle is digenetic. 9. Both male and female forms coil around each other. 10. *Culex fatigans* and *Culex pipiens* serve as the secondary host for *Wuchereria*. 11. Female *Wuchereria* gives birth to sheathed embryos called microfilariae. 12. Microfilariae are long slender forms enclosed by a hyaline sheath. 13. Microfilariae come into superficial blood circulation at midnight and are transferred in the body of the mosquito (*Culex*) during a bite. 14. Microfilariae become sausage-shaped larvae in the thoracic muscles of mosquitos. 15. Microfilariae undergo two moulting in the thoracic muscles of mosquitos and become long slender forms. ### 3. Ancylostoma (Hook Worm) * **Phylum:** Nemathelminthes * **Class:** Nematoda * **Order:** Spongyfloidea * **Genus:** Ancylostoma * **Species:** duodenale **Comments** 1. *Ancylostoma* is commonly known as the hookworm, as it is an endoparasite in the intestine of humans. It is white or ivory in color. 2. The body is narrow, cylindrical, with three lips enclosing the mouth. 3. Denticulate chitinous plates are found in the buccal cavity, which enable the worm to draw blood from intestinal blood vessels. 4. The male is shorter (8-11 mm) in comparison with the female (10-13 mm) in length. 5. The posterior end of the male is provided with an umbrella-like bursa (copulatory organ), while the posterior end of the female is rounded. 6. The digestive system is well developed, having all the parts like mouth, buccal cavity, oesophagus, intestine, and rectum. Female possesses an anus while male has a cloacal aperture. 7. Anteriorly on the ventral side, the excretory pore is present. 8. Reproductive organs are unpaired, testes in male and paired ovarian tubes in the female. 9. These worms take the mucous membrane and blood cells of the host as their nourishment. 10. They cause anemia in children. They also cause mental retardation and weak resistance in adults and children. ### 4. Trichinella (Whip Worm) * **Phylum:** Nemathelminthes * **Class:** Nematoda * **Order:** Trichinelloidea * **Genus:** Trichinella * **Species:** spiralis **Comments** 1. The worm is worldwide in distribution, leading a parasitic life in the intestine of humans, pigs, rats, and other vertebrates. 2. The worm is sexually dimorphic. The male is smaller in size (1.4 to 1.6 mm in length) with a pointed anterior end and a fleshy posterior end. The posterior end is provided with large papillae and copulatory lobes. 3. Lips around the mouth are absent. The pharynx opens into the oesophagus, embedded in a granular mass of cells known as stichosome. 4. Reproductive organs are a single testis and a single ovary in males and females, respectively. The vulva is situated anteriorly. 5. These worms reach the stomach of humans in the encysted form whenever undercooked infected porkmeat is eaten. In the stomach, the cyst is digested, and male and female worms get released. 6. The female gets fertilized by the male just after three days of ingestion. After this, the male dies. 7. Females burrow deeply in the intestinal wall after fertilization, and then on the fifth day, they begin to produce small worms (juveniles). These juveniles reach into skeletal muscles, for example diaphragm, intercostals, muscles of throat, tongue, etc. by lymphatic and blood streams of the host. 8. These worms now get encysted within these cells. It is the critical phase of this disease, which may cause pneumonia, kidney failure, heart inflammation, and toxemia. 9. The cyst of the worms slowly gets calcified after several months. This calcification is completed in 18 months approximately. This calcified cyst makes the worms enable to live in the host for several years. 10. The disease caused by this worm is known as Trichinosis. ## Phylum-Platyhelminthes ### 1. Fasciola hepatica * **Phylum:** Platyhelminthes * **Class:** Trematoda * **Order:** Digenea * **Genus:** Fasciola * **Species:** hepatica **Comments** 1. *Fasciola hepatica* is an endoparasite of the bileduct of sheep and is commonly known as the liver fluke. 2. It causes Liver Rot in sheep. 3. The body is leaf-like, having two suckers: * **anterior sucker (oral sucker):** for food sucking * **posterior sucker (Acetabulum or ventral sucker):** for attachment 4. Suctorial pharynx with a bifurcated intestine. 5. Mouth is present but anus is absent. 6. Intestinal caecae are present to store more and more food. 7. Flame cells for excretion. 8. Mesenchyme is mesodermal in origin. 9. *Fasciola hepatica* is monoecious, i.e. male and female reproductive organs are in the same individual. 10. The testes are two in number. 11. The ovary is single. 12. Fertilization is internal and cross-type. 13. Laurer's canal develops at the time of breeding to receive the sperms. 14. Vitelline glands are yolk glands, which also produce the shell. 15. The shell gland secretes the secretion which hardens the shell. 16. Development starts in the shelled egg. 17. The life cycle is digenetic. 18. Different larval forms are produced during the life cycle. 19. The primary host is sheep. 20. The secondary host is a snail. 21. Larval forms are: * Miracidium * Sporocyst * Redia * Cercaria * Metacercaria 22. Miracidium and cercaria larva are free swimming forms in water. 23. Redia and sporocyst are formed in the snail. ### 2. Miracidium Larva **Comments** 1. It is conical in shape, having a large number of cilia all over the body. Cilia help in swimming in the water. 2. Towards the anterior end, a small apical papilla is present. In the apical region, multinucleated apical gland and unicellular penetrating glands are present, which open outside through the papilla. 3. The ectoderm is made up of an epithelial lining of hexagonal cells. 4. Below the ectoderm, a subepithelial layer's circular muscles and longitudinal muscles are present. 5. Anteriorly, the brain and an eye spot are present. 6. Flame cells with a blind duct and a large number of germ cells are also present. ### 3. Sporocyst Larva **Comments** 1. It is a sac-like structure covered by a cuticle. 2. The cilia and epidermal cells are lost. 3. The eye spot, brain, glands, etc. degenerate. 4. The sub-epithelial cells, muscles, and mesenchyme are as such and germ balls and protonephridia are present. 5. The germ balls produce daughter sporocysts, which again produce the third type of larva, the redia. ### 4. Redia Larva **Comments** 1. It is an elongated sac-like larva, which comes out by rupturing the cyst and migrates to the liver or digestive gland of the snail. 2. A small mouth is present at the anterior end. 3. The pharynx is muscular and the enteron is small. 4. A ring-like muscular collar is present around the anterior region, which helps in locomotion. 5. There is an aperture in the side wall near the collar, which is known as the birth pore. 6. Lobe-like ventral processes, lappets are present in the posterior region. 7. The flame cells form the branching excretory tubules. 8. During favorable conditions, the redia forms daughter redia. 9. Germ balls, after development give rise to the new larval form known as cercaria. ### 5. Cercaria Larva **Comments** 1. It is a heart-shaped larva with a tail, which resembles the adult fluke in appearance. 2. The body is covered by a thin cuticle. 3. The rudiments of most of the adult organs are present. 4. There is a mouth surrounded by an oral sucker. 5. The ventral sucker or posterior sucker (acetabulum) is present on the ventral side. 6. The pharynx is suctorial and the intestine is bifid. 7. Paired excretory ducts with flame cells and genital rudiments are present. 8. Cystogenous cells are the characteristic feature, which are responsible for the formation of the cyst. ### 6. Metacercaria **Comments** 1. It is similar to cercaria, but the tail and cystogenous cells are absent. 2. A cyst is present around it. 3. Flame cells have increased in number, and the excretory bladder opens outside, by an aperture. ### 7. Taenia Solium * **Phylum:** Platyhelminthes * **Class:** Cestoda * **Order:** Taenoidea * **Genus:** Taenia * **Species:** solium **Comments** 1. *Taenia solium* is commonly known as the pork-tape-worm. 2. *Taenia solium* is an endoparasite of the intestine of humans. 3. The body of Taenia is divisible into: * scolex * neck * strobilla 4. The scolex is meant for attachment. 5. The scolex bears suckers and a rostellum. 6. The rostellum has two rows of hooks. 7. The suckers are four in number. 8. Both the rostellum and suckers are used for attachment. 9. The neck is the region of proliferation of new proglottids. 10. The strobilla is a long tapering part, having a large number of proglottids. 11. The strobilla consists of three types of proglottids: * immature proglottids - anterior * mature proglottids - middle * gravid proglottids - posterior 12. Proglottids of the anterior region, after the neck, have a follicular stage and vitelline glands. 13. Proglottids of the anterior region (200) bear testes, but no ovaries, except a few anterior most. 14. Mature proglottids of the middle region have a complete set of male and female reproductive organs. 15. Gravid proglottids of the posterior region contain only a uterus with fertilized eggs. 16. Gravid proglottids detach from the body by apolysis. 17. The tapeworm is acoelomate. 18. The digestive system is simple, without an alimentary canal. Food is absorbed through the body surface. 19. Respiration is anaerobic in *Taenia solium*. 20. Flame cells (solenocytes) are excretory in function. 21. The nervous system consists of an anterior brain complex and ten longitudinal nerves. 22. The tapeworm is hermaphrodite and protandrous. The life cycle is digenetic. 23. Fertilization is internal, cross-type between proglottids. 24. Testes are many in number (nearly 200 in number). 25. Ovaries are two in number. 26. Development (cleavage) starts in the uterus in the gravid proglottid in the intestine of humans. ### 8. Hexacanth **Comments** 1. The hexacanth is developed in a shell with six hooks (canth or teeth). 2. The hexacanth stage is the infective stage to the pig: * humans are the primary host * pigs are the secondary host 3. The hexacanth bores the intestine of the pig and comes into the blood circulation. 4. Teeth are lost, and it reaches the muscle. 5. In muscle, a bladderworm is formed. 6. The bladderworm becomes encysted and is known as cysticercus 7. Cysticercus is the infective stage to humans. 8. Infection is transmitted by measly pork. 9. In the intestine of humans, the cyst is broken and the bladderworm becomes free with an everted scolex. 10. It attaches with the wall of the intestine, and the bladder is detached. 11. New proglottids are strobilized, and the adult taenia is formed. 12. The onchosphere is the larva of Taenia, also known as hexacanth. ### 9. Bladderworm **Comments** 1. The bladderworm is the larva of Taenia, also known as cysticercus. 2. Cysticercus is formed by invagination and formation of the proscolex. 3. During infection by Taenia, necrosis of the brain and epilepsy may appear. 4. The vitelline gland is the yolk gland, which also produces the shell. 5. The shell gland hardens the shell. 6. Excretory waste is ammonia, fatty acids. ### 10. Echinococcus Granulosus * **Phylum:** Platyhelminthes * **Class:** Cestoda * **Order:** Echinococcus * **Species:** granulosus **Comments** 1. It is an endoparasitic worm commonly found in the small intestine of dogs, cats, and other carnivores. 2. The body measures about 2 to 8 mm in length. It can be divided into the scolex (head), neck, and 3 to 4 successive segments known as proglottids. 3. Four adhesive suckers are present in the scolex, which is a muscular structure. 4. The small neck shows strobilization. 5. A double row of 30 to 36 hooks is present on the retractile rostellum. 6. Among the three proglottids, the anterior one is immature, the middle is mature, and the last one is gravid. 7. The mature proglottid possesses a mature male as well as female reproductive organs. The male organs are testes, vasa deferentia, cirrus, and a cirrus sac, while the female organs are a pair of ovaries, oviducts, vitellaria, ootype, uterus containing eggs, and a vagina. 8. In mature proglottids, a germinal mass is present, which gives rise to genitalia. 9. In humans, it is found in the form of a bladder, known as a hydrated cyst. It gives rise to fatal, cancer-like tumors. ### 11. Schistosoma * **Phylum:** Platyhelminthes * **Class:** Trimatoda * **Order:** Digenes * **Genus:** Schistosoma * **Species:** haematobium **Comments** 1. *Schistosoma* is commonly known as the blood fluke and is found in the blood vessels and hepatic portal system of humans, cats, pigs, dogs, etc. 2. This endoparasite causes a disease called Schistosomiasis. 3. The phenomenon of sexual dimorphism occurs. Thus, males and females are separate, but they live in close association. 4. The male is flattened, while the female is slender. Both possess oral and ventral suckers. 5. The mouth, which is surrounded by an oral sucker, leads into the oesophagus, which, on its turn leads into a forked intestine. The pharynx is absent. 6. The ventral folding from the male's body forms a groove, known as the gynecophoric canal, in which the female individual lives. 7. In case of the male, reproductive organs are four or five testes, a vas deferens, and a seminal vesicle, while female organs are an unpaired elongated ovary, oviduct, vitelline glands, and uterus. 8. After fertilization, the egg develops into the miracidium larva, which is free swimming. Later on, it penetrates the snail's body and gets converted into the cercaria larva. 9. The infection in humans occurs when the cercaria penetrates the skin, followed by pain, itching, etc.