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Physiology of Trematodes


The trematodes are multicellular, endoparasite. They belong to Platyhelminthes phylum. The normal body physiology of trematode is explained below;

  • Nervous Systems

In the evolutionary history they are the first organisms which have the central nervous system. The nervous system of trematode is able to produce the different kind of substances which directly influenced its growth and development these substances are called neurosecretions. It has almost all major kind of neurotransmitters that are present in vertebrates. These secretions researched to their destination in the form of synaptic vesicles and through the paracrine system. All the body processes which need to be regulated for a long period of time like growth or development are regulated by the nervous system. As there is no circulatory system in trematodes so these secretions are delivered by nervous system in the intercellular spaces of the desired region.

The nervous system of trematode is comparatively highly secretory. Mostly neurosecretory cells present in the anterior region in ganglia. The neuro secretions of trematodes changes from time to time. Some neurotransmitters are inhibitor like ACH and some are activator like dopamine. It is also absorbed that trematode can absorb these neurotransmitters from host through tegument. These changes may be due to the change in the host environment. The neurosecretions decrease with the growth of trematodes

Information gathering: Trematodes have chemosensory, mechanosensory and photosensory apparatus some may have cilia and some may not have cilia. They may present on the tegument or below the tegument surface Most out of them are present in anterior region and some out of them are present in dorsal region. These sensory organs help the trematodes to detect the changes in his surrounding environment. The cilia like sensory organs present on tegument collect information from the environment and tell the trematode what is the right time to do something e.g. When metacercaria reach in the intestine then its sensory organs tell him that it is the right for excystation. The development of any system in trematode and growth is governed by these sensory organs. On receiving a stimulus from environment and from within the host these organs stimulate the neurosecretory cells which produced hormones that play a role in development, growth, digestion and on every other system

There is papilla like the taste organ present near the mouth of trematode that helps the trematode in the selection of food

In some species miracidium and sporocyst contain two eyes like structure and in some species the larva may have a third eye. These organs also have a role in sensing the surrounding environment. These eye like structures act like the photosensory organs which help them to find out their hosts

Structure of Trematode's Nervous System


Digestive system of trematodes

The trematodes have sucker which help them in attachment and in sucking food. After sucking food reached in the muscular pharynx which push the food into esophagus and ceca. The ability of pharynx to push the food is due to muscular ring around the pharynx. The esophagus and pharynx regions have digestive glands which secret enzyme. The esophagus receives food from pharynx and it has projections which are projected toward the lumen this thing increases surface area of the esophagus. These thread-like extensions entangle with food particles this increase their interaction time with the enzymes that are secreted in the esophagus.

The ceca received food from the esophagus. Its epithelium lining is capable to do both absorptive and secretive functions. When trematode newly excyst, at that time its epithelium is secretive in nature which help it in penetration of mucosa to reach its desired place. The secretions of ceca also played a role in the maintenance of glycocalyx

Development of digested system: The first developmental stage of trematode in which it develops is Radii and it is absent in miracidium which are non-feeding stages and sporocyst after that this system keep on growing and develop in lateral stages like cercariae, meta cercaria and in adult. neurosecretory hormones are responsible for the development of digestive system

Feeding: Trematode required which type of enzyme is depended upon which type of food is being ingest by trematode. Some trematode lives in the host intestine where they attach on surface epithelium and absorb host digested food in this case, they don’t need to digest it properly because it is already digest and they easily absorb this food. on the other hand, there are some other parasites which penetrate the liver parenchyma like fesciola, bile duct likes prosthogonimus, lungs, heart and blood vessels these feed on tissue, blood, and bile

Digestive glands and their secretions: The digestive glands are present in pharyngeal and esophageal regions and in the epithelium of ceca. These glands produced lipase, aminopeptidase, esterase, alkaline phosphatase, ATPase, protease these enzymes are secreted and poured into ceca where it digests the ingesta and convert them into small molecules which are absorbed and then assimilated. Some trematodes remain in intestine while some penetrates the mucosa and reach into liver, lungs, heart and blood vessels. The newly excysted trematode has ceca which is secretory in nature which help the trematode to penetrate the mucosa and reached the desire organ.

Digestion: Mostly trematode ingest food through mouth and the digestion occur extracellular but it also occurs intracellularly. In plug feeders and parasites that ingest sold food like fesciola, digestion is extracellular while those that feed on fluids like schistomoases species digestion is predominantly intracellular. It may absorb micro food particles or already digested food from host. The absorption through tegument occurs through active transport and this all happen across the concentration gradient which requires energy the molecules like glucose, amino acid and other product of the digestive system of host absorbed by tegument. The parasite which don’t have a gut absorbed all nutrients through tegument. When food absorbed through tegument then the food doesn’t go into the digestive track because it is already digested by host digestive enzymes so it is readily absorbed and available for use

Structure of Digestive System


Excretory System

The excretory system also serves as an osmoregulatory mechanism. It maintains the osmotic pressure inside the trematode. In the tubular network there are the large number of flame cells which are equipped with cilia which beat in a specific pattern. The number and arrangement of those cell are so specific and special that you can classify trematodes on the bases of these cells’ arrangement and morphology.

When cilia beat fluid move in the terminal ducts due to the movement of cilia. Many terminal ducts join together to form a large collecting duct which takes the fluid from terminal ducts and drain it into urinary blood. The urinary bladder presents posteriorly between the two ceca. The urinary bladder open to outside through an excretory pore which is present at posterior end.

In some species collecting ducts have microvilli and alkaline phosphate which are required for reabsorption of wastes from the collecting ducts

The main nitrogenous wastes are ammonia and urea however there are other soluble wastes also present

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Excretion can be done through the movement of wastes like ammonia, amino acid and h+ through the tegument. The movement through the tegument is regulated by Na-K when the external environment of organism become acidic then NA+ levels in the tegument increase which help the organism to manage the acid load.

Lymphatic system:

It may be well developed in some species and in some other species it may be poorly develop. It has three canals that run across the body which are present in dorsal region, ventral region and in lateral region of the body. These tubes like other organs are embedded in parenchyma. These canals performed the function of drainage. In this way lymphatic system play an important role in osmoregulation

Excretory Mechanism


Reproductive System

Trematodes are hermaphrodite except schistosomes which have separate sex. The cross and self both type of fertilization occur in trematodes. They find each other through chemoattractant like cholesterol and steroid except schistosomes. Asexual reproduction occurs by parthenogenesis in some developmental stages of trematodes.

Male reproductive system:

The main function of male reproductive system is to fertilize the eggs but in trematodes it is also responsible for the growth and development of female reproductive system. Studies shows that without male reproductive system the female reproductive system is not able to produced eggs

Prostate gland: It has the two type of secretions. One has glycoproteins granules and other type have both protein and phospholipid granules. They are surrounded by connective tissue and they add their products in ejaculatory duct. Its main functions are stimulation of female reproductive track, providing fluid medium and energy to sperm and ensure the modality of sperms

Spermatogenesis: it is the process of sperm formation. It takes place inside seminiferous tubules present in testis. Testis have the two type of cells one is germinal which form sperms and other type is nongerminal which get nutrition from parenchyma and use it for sperm formation. It includes following steps; primary spermatocytes convert into secondary spermatocyte which develop into tertiary spermatocyte and it transforms into spermatid and finally spermatozoa. Spermiogenesis is the last step of spermatogenesis. In it the differentiation zone forms in spermatid which are separated by microtubules. They contain nucleus, mitochondria and a pair of centrioles which develop in two flagella laterally there is a median cytoplasmic process which fuse with flagella like centrioles. Spermatogenesis completed with constriction at differentiation zones and many newly formed spermatozoa move out. A sperm is formed in 6 days. The spermatogenesis duration is also influenced by temperature. As first sperm release in the bundle of 32 after they became separate from each other and move in vas efference.

Female reproductive system:

The main function of female reproductive system is the production of oocyte, provide an area for fertilization, formation of egg shell and laying down eggs.

oocyte formed in ovary and then pass out into oviduct and this movement is regulated by a sphincter. The oviduct has a structure associated with it called seminal receptacle which store male sperms which are used to fertilize the egg

Laurer canal present in the seminal receptacle it may end in parenchyma, blindly or it may open to outside through tegument. In some species it is used for copulation and It is also supposed that it is used to dispose of residual sperms and wastes of egg formation


The female reproduce system has two glands which are vitelline gland and Mehlis gland their secretions are important for egg nutrition and egg shell formation. At first Mehlis gland is activated which followed by the activation of vitelline gland

Mehlis gland: These glands are present around the oviduct. These are unicellular glands and pored their product through a duct into oviduct. Their secretions are lipoprotein in nature and form a template of egg shell on which the egg shell material can be attached. These are responsible for egg shell formation, hardening of shell, activation of the sperms. They also activate the vitelline gland to release its secretions. It plays a role in stimulation of spermatozoa and lubrication of the uterus. It may facilitate the penetration of sperms in some way

Vitelline gland: These glands present close to tegument which ensure that vitelline glands should get a continuous nutrient supply. when sperm penetrates the oocyte then vitelline gland which are present laterally, pored the vitelline gland cells into oviduct which are important for nutriment and egg shell formation. The egg shall make up of different types of protein which are provided in the form of vitelline cells

Oogenesis: it is the process of the formation of oocyte. It occurs inside the ovary. Its stages include the formation of oogonia, primary oocyte, secondary oocyte, pronuclei and then finally fertilization occur. In this reduction occur in the first mitotic division in primary oocyte

Copulation: The schistosomiasis species in which both sexes are separate. Male and female find each other through chemoattractants which are lipid like cholesterol. The Male grab the female at the level of vagina then circus protrude out and moved in the vagina where male ejaculate and sperm travel the uterus and reach the oviduct.

Egg: The formation of the egg required four things first one is oocyte second is sperm third is vitelline cells and forth is egg shell. At one time only one egg form in ootype after its formation it moved into uterus and the formation of second egg started in the ootype. The ootype of each trematode has a particular shape and when sclerotin present in the shell of oocyte harden in ootype the shell of egg acquired the shape of ootype that’s why the egg of a specific trematode has a specific shape. Its outer covering is a hard shell inside of it there is a vitelline membrane which contain nutritional substance and miracidium. the egg has operculum at its one end this is a lid like structure which is used by miracidium to come out of the egg

Parthenogenesis: The parasite shows this phenomenon in their intermediate host. At that time his sexual reproductive system is not fully develop so reproduce asexually to increase their generation. In this case an unfertilized ovum develops into new individual.


Developmental stages

Its life cycle starts with the egg. Eggs came out with fences from the final host. In some species egg hatch within the environment when they find suitable conditions while in some species it hatches with in the host.

When egg hatch miracidium come out and it is a nonfeeding stage of parasite. It is ciliated which just that it can survive in water bodies. It is not a infective stage. Miracidium enter in the body of intermediate host

In intermediate host miracidium develop into sporocyst. This stage of trematode cannot live in environment. The mother sporocyst produced daughter sporocyst and this all happened by Asexual reproduction. There are usually two generation of sporocyst inside I.H. It does not have a digestive track. It absorbs nutrients through tegument

The sporocyst develop into radia. It has a mouth a digestive system. It is also present inside the I.H. there may be a competition between sporocyst and radia as both are living inside the same host

Radia develop into cercaria. The cercaria can develop directly from sporocyst it depends upon the specie. It is an immature fluke. Some species have tail some don’t have tail. Its head is tapered and penetration gland also formed which produced creations that help in penetration. It is an infective stage. Its excretory system also developed

Cercaria may develop into metacercaria which is actually an encysted form of cercaria it may form on vegetation and inside the I.H muscles. The morphological changes may occur in metacercaria It also causes infection in final host.

When cercaria or meta cercaria reached inside the final host then it develops into the adult. Adult is capable to reproduce inside the final host and produced eggs. Its digestive and nervous system is developed



All trematodes at some point of there life cycle have anerobic metabolism and at some stages aerobic. The free-living stages of the trematodes e.g. stage forms, when a trematode leave a host and waiting for his entry into another host in that case, they show aerobic metabolism because they are present in environment and free oxygen is present. This show that which type of metabolism trematodes perform not only depend on internal enzymes but it also depends on the external environment. In encysted metacercaria wastes can not be moved out of the body so their metabolism is modified in that way that they produce insoluble metabolites which is stored in the bladder. The metabolism models of Schistosoma and fesciola are two different models as their nutrition values are totally different. The Schistosoma live mesenteric veins which are rich in nutrients while fesciola live in liver which have comparatively low nutritional values.

The metabolism also depends on needs of an organism e.g. male have high glucose levels because they required energy for copulation while females have high protein levels because they use protein in egg shell formation.

Musculature of Trematodes

The musculature of trematode consists of smooth muscle cells. As trematode lack body cavity so its organs are embedded in connective tissue. You can divide this muscular system into two parts. The first part is present just below the tegument and this part is called sub-tegumental musculature. This play a main role in locomotion of parasite. In adult of most the species these muscles are not very strong. The second part or zone of this system is present around the gastrodermis. This zone helps the organism to move the content in GIT track. The anterior region of trematodes is more muscular than any other region of body.


it is an outer covering of trematodes. It is metabolically is multinucleated with no cell boundaries. Its outer coat is glycocalyx. Its functions are

interaction with host

protection from host digestive enzyme

abortion of molecules

gaseous exchanges

sense the surrounding environment

influence the neurosecretions

ATP synthesis

removal of waste

anerobic respiration

the tegument is the surface of the trematode that interact with the host. Most of the parasites live in the intestine of their host where there are numbers of digestive enzymes present. If a parasite wants to survive there then it has to neutralize the effect of these enzymes and this task is performed by tegument.

Some parasites don’t have gut so they have to absorb food from the body surface and this task is performed by tegument even those trematodes which have gut has to absorb glucose and amino acid and this tasked is performed by tegument and all happen against the concentration gradient means it requires energy.

Mostly all trematode developmental stages use oxygen however adults are facultative anaerobe mean they can survive in low or no oxygen. These oxygen requirements are fulfilled by tegument because trematodes don’t have any organ for respiration

Trematodes also contain mitochondrias which help in cellular respiration and as a result ATP synthesized which is to absorb the food across the concentration gradient. The mitochondria used environmental oxygen in cellular respiration and produced ATP.

The movement of waste, nutrition and gases through the tegument is regulated by Na-k ATPase pump. Through this the Na concentration in the tegument is managed which to manage the charge on the tegument. This thing gives the ability to trematode to absorbed things from outside and remove wastes from inside

The developmental stages of trematode are directly related to its outer environment. When the outer environment of trematode changes it transforms into another form. The tegument helps the trematode to sense these changes. There are sensory cilia present in the tegument. These cilia collect the sensory information from the surrounding and send it to this way tegument play roll in development and for development neurohormones required which are secreted by neurosecretory cells in brain so in that way tegument also regulate the production of neurohormones

The tegument may carries antigens which provoke the host immune system and as a result immune response starts and the concentration of immune cells in blood increases.


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