Platyhelminthes

Introduction

The phylum Platyhelminthes contains both parasitic worms and the free living flatworms. These flattened wormlike organisms range from a millimeter to centimeters or rarely meters. The mouth is typically the only opening to the digestive tract, they use the same opening to both ingest food and egest waste. In both of these groups, protonephridia are present for excretion of wastes and individuals are hermaphroditic.

Parasitic - Parasitic Platyhelminthes

Introduction

All of these groups of Platyhelminthes are parasitic and have adapted their life cycles and body structures to help them better utilize their hosts.

Cestoda
Introduction

Cestoids are generally known as tapeworms and all species are endoparasites in the guts of all types of vertebrates. They are quite diverse with 3400 species worldwide.

Morphology

Cestoids are known as tapeworms because that is what they look like; long and thin much like an unravelled role of tape. Adults are dorso-ventrally flattened but unlike other flatworms possess is an anterior, suckered head region called a scolex for attachment to their host. The scolex is often minute compared to the rest of the body and is generally a four sided knob with suckers or hooks for attachment to the host gut wall. Behind the scolex is the narrow neck which is proliferative(produces all each segment) and gives rise to the body segments called strobila. The strobila consists of linearly arranged sectional segments called proglottids.

Cilia are used to locomote in some dispersal stages of the life cycle but adults are parasites they only need to move in the digestive tract of their host. Any movement is accomplished by transverse, longitudinal and circular muscle movements.

The cestodes have very limited sensory abilities due to their parasitic existence. The adults have virtually no sensory organs and the oncosphere has only cilia with which to detect its environment.

Metabolism

Since their body is small and dorso-ventrally flattened, there is no need for any sort of respiratory system. The diffusion distance for gas exchange is short and oxygen is absorbed across the general body wall. The protonephridial system extends through the chain of proglottids and drain into collecting canals finally, emptying into the exterior.

Reproduction

Cestodes are hermaphroditic with a complete reproductive system on each proglottid. At copulation the cirrus of the male is everted into the vaginal opening of the proglottid of an adjacent worm. Zygotes remain in a blind-ended uterus where development begins and the egg capsule hardens. In many cases the terminal proglottid packed with eggs breaks away from the strobila and is freed within the feces

The life cycles of cestodes vary in their requirements for number of intermediate hosts. In the aquatic food chain, species are widely distributed in northern areas and are parasitic in the guts of many fish-eating carnivores including humans. Species of Diphyllobothrium possess egg capsules which are deposited with the faeces in water and a ciliated, free swimming coracidium bearing six small hooks, hatches in 10 days. When the coracidium is ingested by a copepod crustacean it penetrates their gut wall and develops into a procercoid. When the copepod is eaten by a fish the procercoid migrates to striated muscle of the fish where it transforms into the plerocercoid stage. The plerocercoid develops into an adult tapeworm when the fish is ingested by a suitable warm-blooded host.

Ecology

Cestodes are found in the gut of many species which consume fish (see life cycle). The head or scolex is adorned with hooks and suckers that enable the cestode to attach to the gut lining. Here they feed upon tissue and blood cells.

Idiosyncratic Inverts

Tape worms are extremely long and can grow to a length of 15 meters.

Digenea
Introduction

In terms of parasite abundance, digeneans are second only to nematodes. Digeneans have complex life cycles, and usually have a mollusc as one of their intermediate hosts.

Morphology

The body forms of digeneans vary during life but adults range from 0.2 mm to 6.0 cm in length. They are typically dorso-ventrally flattened with an oral sucker surrounding the mouth, although some species have a mid-ventral or posterior ventral sucker as well.

When in the egg stage they are at the mercy of water currents, but once they hatch into miracidium they are able to locomote using cilia. During the cercaria stage they develop both a tail which enables them to find a host and a digestive tract and suckers. Locomotion is only required when looking for a host.

The surface of the body has a variety of sensory papillae and ocelli occur in many miracidia and some cercariae stages.

Metabolism

Since the body is small and dorso-ventrally flattened there is no need for any sort of respiration system. The diffusion distance for gas exchange is short and oxygen is absorbed across the general body wall.

Digeneans, like most other Platyhelminthes, have no special provisions for circulation but do have a lymphatic system. Canals lined with flattened cells are interspersed around the muscular organs and have free cells floating in the enclosed fluid.

Digeneans have protonephridia as well as flame cells which they use to excrete nitrogenous wastes and to disperse the wastes.

Reproduction

All digeneans are hermaphroditic and produce huge numbers of eggs, 10,000 to 100,000 times greater than that of free-living flat worms. They are all hermaphroditic. The male organs consists of two testes which connect to sperm ducts that unite and lead to an eversible copulatory cirrus. The cirrus protrudes into a common genital atrium. The female organs consist of a single ovary and oviduct which leads into a small sac called the ootype. It is surrounded by a gland which is important in egg capsule formation. The ootype leads into the uterus which runs into the genital atrium and gonopore. During copulation sperm exchange is mutual and usually dependent on cross-fertilization although self-fertilization does occur. Sperm travel up the uterus and are stored in the seminal receptacle. Eggs are fertilized in the oviduct or ootype and the egg capsule that develops pass along the uterus where embryonic development begins and eventual release into the environment.

The life cycle of digeneans varies among species but all species have two infective stages which infect two different hosts. The life cycle begins when an adult digenean releases eggs which pass out of its host, usually through the faeces. The eggs hatch into a ciliated miracidium which actively seeks out the first host, a gastropod, penetrating its skin and metamorphosing into a saclike sporocyst. In this stage it grows into a cercaria which possesses a single opening digestive tract, suckers and a tail, enabling it to leave the snail and seek out the secondary intermediate host, usually an arthropod or a fish, where it encysts, becoming a metacercaria. After encysting in the gut, or gut derivatives the metacercaria develops into an adult at which time it reproduces, releasing more eggs to continue the cycle.

Ecology

All digeneans are parasites of at least two host organisms at various stages of their life cycle. Once within a host they feed on its body cells, blood cells, mucus or tissue exudates. Host tissues are injured by the strong oral sucker that the organism uses for attachment or by the enzymes secreted used to soften host tissues to prepare them for digestion.

A bulbous pharynx ingests food into a short esophagus that divides into two blind intestinal caeca that extend posteriorly along the length of the body.

Idiosyncratic Inverts

The schistoma larvae of one digenean cause the skin irritation, swimmer's itch, in humans. The larval metacercaria penetrates the skin of individuals and die, causing an allergic reaction in sensitive individuals. Human penetration by the larva is purely accidental, the larva was really seeking a bird host.

Monogenea
Introduction

Monogeneans are parasites of aquatic vertebrates, usually fishes, but sometimes amphibians and reptiles. Their bodies are dorso-ventrally flattened and have a large, posterior attachment organ called a haptor which bears hooks and suckers allowing them to cling tenaciously to their host. The possess a single life cycle unlike digeneans, in that there is no intermediate host.

Morphology

The body is compose of a head, trunk, and haptor. The head lacks an oral sucker but adhesive glands are present. The haptor is a single disc with one to three pairs of large hooks and many marginal hooklets.

When the parasite is not attached to its host, it is able to locomote using an inch worm-like motion with the aid of its oral adhesive glands and haptor. During the larval stages it is able to move around using the coordinated movement of cilia covering the body.

Ocelli are found in the juvenile oncomiracidium stage but are usually been lost in the adult due to their parasitic behavior.
Metabolism

Since the body is small and dorso-ventrally flattened there is no need for any sort of respiration system. The diffusion distance for gas exchange is short and oxygen is absorbed across the general body wall.

Monogeneans have protonephridia consisting of scattered terminal cells and their collecting tubules to filter wastes from the organism. These tubules open to the exterior at two dorso-lateral pores and flame cells.

Reproduction

Monogeneans have only one intermediate host in their life cycle, which can either be a fish or an amphibian. Their egg capsules are released into the water and an oncomiracidium develops which attaches to the gills of a tadpole or a young fish where it develops into an adult. Monogenans have complex, hermaphroditic, reproductive systems. The male reproductive system has a single, oval testis occasionally paired, with a sperm duct that leads to the copulatory structure. This is typically a protrusible penis-like structure that may be armed with hooks. During copulation there is mutual exchange of sperm, passed into the single or paired vagina of the partner. Sperm is stored in the seminal receptacle near the single ovary and extensive vitellaria.

Ecology

Monogeneans are parasitic with only one intermediate host (see life cycle). The digestive system of monogenan is similar to that of digeneans but the pharynx secretes protease that digests the host's skin, allowing the parasite to ingest blood and cellular debris.

Idiosyncratic Inverts

Members of one genera of monogenean, Polystoma, lives in the urinary bladders of frogs. Larvae attach to tadpoles and upon its metamorphosis move into its urinary bladder, lose their larval characteristics and become sexually mature.

Non-Parasitic - Turbellaria

Introduction

Scientists can train turbellarians to do simple tasks, such as finding water in a maze. When these flatworms are then cut in half, the new halves learn faster than the original.

Morphology

Turbellarians are dorso-ventrally flattened and vary in shape from oval to elongate. They are usually brown, black or grey in colour and range in size from a mm to more than 60cm. All turbellarians possess an intestine without an anus and the mouth is used for both ingestion and egestion. Smaller turbellarians have a simple unbranched gut while larger species have guts with lateral diverticula that greatly increase the surface area for absorbing nutrients.

Small turbellarians use waves of ciliary action for locomotion to glide over surfaces, whereas larger species use muscular movements of their entire body to creep, swim, twist or somersault along the substrate. Contractions of the body wall are accomplished by outer circular muscles, inner longitudinal muscles and in some of the larger species additional diagonal muscles.

Turbellarians have a vast array of organs that enable them to sense environmental stimuli. These organs include sensory hairs, eyes, statocysts and chemoreceptors of various types.

Some species possess ciliated pits in front of their cerebral ganglion that are used in phototaxis (movement towards light). Turbellarians usually have one pair of direct or inverted cup eyes for detecting movement. As well as immobilizing prey, poisonous rhabdoids, rod-like structures produced in the epidermis, on the front external surface also help deter potential predators.

Metabolism

Since their body is small and dorso-ventrally flattened there is no need for any sort of respiration system. The diffusion distance for gas exchange is short and oxygen is absorbed across the general body wall.

Nutrients can ordinarily simply diffuse from the central gut to nearby tissues, but large species possess branches on the gut to aid nutrient transport to the margin of the body.

Turbellarians eliminate nitrogenous wastes by diffusion across the body surface, and excrete excess water and other waste metabolites using protonephridia. These are distributed over the body of the turbellarian, and collect waste and feed it into ducts which eventually lead to the exterior through pores. At the end of each duct there are special ciliated structures called flame cells which help to disperse wastes away from the organism.

Reproduction

Turbellarians are hermaphroditic and typically reproduce both asexually and sexually. The majority of sexual reproduction in turbellarians is hermaphroditic by way of copulation and fertilization is generally internal. The male and female systems are complex but show some general features. The male part of the system consists of paired testes leading into a sperm duct, a storage sac or seminal vesicle and a penis. The female part of the system is specialized into the gonopore (vagina), copulatory bursa and seminal receptacle for short and long-term sperm storage, respectively. The female system also produces eggs in the ovaries and transports via the oviduct to the gonopore.

Freshwater turbellarians frequently use asexual reproduction in times of environmental stress and when mating partners are scarce. This is usually accomplished, by one of two methods: budding and transverse fission. In budding, the buds (zooids), differentiate along the length of the "parents body", forming chains until fission separates them into new individuals. Transverse fission or division begins with a division forming behind the pharynx and separating the organism into halves. The posterior half of the worm usually attaches itself to the substratum while the anterior half continues to move forward until the two halves pull apart. Each half then regenerates the missing structures to form a complete worm. A few species of freshwater planarians actually fragment their body into several pieces, each of which regenerate to form several small worms.

Ecology

Turbellarians feed predominantly on bacteria, protozoans and other small invertebrates in their benthic habitats. Most turbellarians are carnivores but scavenging is not uncommon. Prey is immobilized by special structures called rhabdoids which enable the turbellarian to swallow the prey whole, in pieces or suck the body fluids out of it.

Idiosyncratic Inverts

Members of one family of turbellarians, Phagocata, simultaneously fragment their body into several pieces and develop a new worm from each piece!

Glossary

Citation

Hebert, P., & Ontario, B. (2007). Platyhelminthes. Retrieved from http://www.eoearth.org/view/article/155266

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