Platyhelminthes

The term Platyhelminthes was first proposed by Gangenbaur (1859), which means flatworms because of their characteristic contour of flattened bodies. The term has been derived from two Greek words: platys, flat, and helminths, which means worms. They are a diverse group comprising about 18500 living species, exhibiting evolutionary achievements over the diploblasts in having a structural body plan based on bilateral symmetry, in having a third layer of cells, the mesoderm and in having definite organs or system of organs. They, however, lack a coelom, the body being compact (acoelomate), and the blood vascular system is absent.

1.0General Characters of Phylum Platyhelminthes

• Triploblastic, acoelomate (without a body cavity) and bilaterally symmetrical animals.
• Body soft, unsegmented and dorsoventrally flattened. 
• Body shows organ grade organization. 
• Body shape varies from moderately elongated flattened to long, flat, ribbon-like, and leaf-like. 
• The length of the body may be highly elongated in some and reach as much as 10 to 15 metres. 
• The metameric segmentation and skeletal structures are absent. Pseudometamerism is seen in some members.

• The anterior part of the body is differentiated into a head.
• The body is covered with a syncytial, one-layered, partly ciliated epidermis, while in parasitic forms (trematodes and cestodes), the outer body covering is a Cuticle. 
• Adhesive structures like hooks, spines and suckers and adhesive secretions are common in parasitic forms. 
• The mouth is the single opening of the digestive tract, and the anus is absent. The digestive tract is absent in some. 
• No respiratory and circulatory systems. Respiration is aerobic in free-living forms but anaerobic in parasitic flatworms—gaseous exchange by diffusion.
• The nervous system is the most primitive type, ladder-like, consisting of a pair of anterior ganglia with longitudinal nerve cords connected by transverse nerves. 
• Sense organs are in the form of eye spots, and chemoreceptors are in free-living forms.  
• The excretory system consists of protonephridia with flame cells. Absent in some primitive forms. 
• The sexes are mostly united, i.e., hermaphrodite or monoecious, but the digenean flukes are gonochoristic (separate sexes). 
• The reproductive system is complex and highly evolved in most forms. Asexual reproduction by fission also occurs in many free-living forms. 
• Fertilization internally may be cross or self-fertilization. 
• Development may be direct or indirect. In some parasitic forms, development is very elaborate, involving several larval stages and hosts. 
• Parthenogenesis and polyembryony are common in trematodes and cestodes. 
• Flatworms are free-living (turbellarian), ecto, endo commensals, or endoparasites. 
• They occur in all major habitats, aquatic and terrestrial, and in the tissues of other animals.

2.0Phylum Platyhelminthes Classification

According to them the phylum Platyhelminthes is divided into four classes. 

• Class Turbellaria  
• Class Trematoda 
• Class Monogenea 
• Class Cestoidea

Class Turbellaria (L. turbella = a little string) 

• Turbellarians are mostly free-living and aquatic. The great majority are marine and mostly benthic. A few are terrestrial but confined to humid areas. Some are brightly coloured. 
• Body size ranges from a few millimetres to 50 centimetres. 
• The body is unsegmented, flattened, and covered with a ciliated cellular or syncytial epidermis containing mucous-secreting cells and rod-shaped bodies called rhabdites. 
• Locomotion by cilia and muscular undulations. 
• Epidermal gland cells help in adhesion, mucous secretion, and other secretory functions. 
• Mouth opening ventral, located at the end of an eversible pharynx, leads into a sac-like lobed or much-branched intestine. Suckers absent. 
• Respiration by body surface. 
• The nervous system consists of several longitudinal nerve cords associated with peripheral nerve nets and cerebral ganglions. 
• Several pigment cups, ocelli and statocysts are the sensory organs. 
• The excretory system includes flame cells in most cases. 
• Mostly hermaphrodite, with internal fertilization. Many reproduce asexually using budding or transverse fission and show high power of regeneration. 
• Development is direct in most species. A free-swimming larval stage Muller's larva is present in some forms (in a few polyclads). 
• Examples: Planeria (freshwater flatworm), Bipalium (land planarian), Oligoclado (marine polyclad flatworm), Bdelloura (commensal on book gills of horseshoe crabs), Dugesia (freshwater flatworm).

Class Trematoda : (Gr. trematodes = perforated/having pores)

• All are parasites, occurring especially in vertebrates. 
• Unsegmented dorsoventrally flattened leaf-like bodies, hence they are called "flukes".  
• The body covering is the cuticle, or tegument, which is thick and protects the parasite against host enzyme action. Cilients and rhabdites are absent. 
• One or more well-developed suckers are present—oral sucker for feeding and Ventral sucker (acetabulum) for attachment. 
• The mouth is anteriorly placed, and the gut is well developed, with a pharynx and branched intestine—excretion by flame cells. 
• Three pairs of longitudinal nerve cords. Sense organs are poorly developed. 
• Mostly monoecious or hermaphrodites. The testes are usually two or many, but always a single ovary. No asexual reproduction. 
• Development is direct (in ectoparasites) or indirect (in endoparasites), alternating hosts involving many larval forms. 
• Examples: Fasciola hepatica (liver fluke), Schistosoma (blood fluke), Aspidogaster, and Cotylapsis.

Class Monogenea : (Gr. monos = single; genos - a race) 

• They are mostly ectoparasites (some endoparasites) of aquatic vertebrates, particularly fishes, amphibians, and reptiles (turtles). 
• The body is dorsoventrally flattened, and the posterior end is provided with a large attachment organ or adhesive disc called opisthaptor or haptor. It bears hooks and suckers, allowing the parasite to cling tenaciously to the skin of the host. 
• The anterior end also has an adhesive organ, called a prohaptor, with adhesive glands and suckers. 
• The gut is present, but the mouth lacks a sucker. The pharynx secretes a protease that digests the host's skin. 
• Inconspicuous protonephridia having paired excretory pores situated anteriorly on the dorsal side.
• All are hermaphrodite. 
• The life cycle is simple with a single host, i.e. no intermediate host. One egg gives rise to one adult worm, hence the name "Monogenea", meaning "one generation".
• Ciliated "oncomiracidium" larva in the life cycle. 
• Examples: Polystoma (in the urinary bladder of frogs and toads), Polystomoidella (in the urinary blade of turtles), Dactylogyrus (on the gills of freshwater fishes), Gyrodactylus.

Class Cestoidea : (Gr. kestos = a girdle, L. cestus = ribbon; eidos = form) 

• All are highly specialized endoparasites of vertebrates and are commonly called tapeworms. 
• The body is flat, elongated, and ribbon-like, covered by a non-ciliated syncytial tegument (cuticle) with microvilli-like projections. Rhabdites are absent. 
• The body is differentiated into three regions-head or scolex, neck and strobila or body. 
• Scolex usually with hooks and suckers for adhesion or attachment to the host.
• The neck is very short and narrow. It is proliferative, giving rise to the body or strobila. 
• Strobila consists of a linearly arranged number of segment-like sections called proglottids. 
• The mouth, digestive tract, and sense organs are absent. 
• Each mature proglottid contains one or two male and female sex organs. Thus, tapeworms are hermaphrodites.
• The life cycle is complicated with one or more intermediate hosts. 10. Embryos and larvae possess hooks. 
• Examples: Taenia solium (pork tapeworm), Taenia saginata (beef tapeworm), Echinococcus granulosus (hookworm), Diphyllobothrium (fish tapeworm), Hymenolepis nana (dwarf tapeworm in the intestine of man).