11th Class Biology Animal Kingdom Phylum Platyhelminthes : The Flat Worms

(Gk. platys = broad or flat; helmin = worm)

Brief History : Aristotle mentioned tapeworms, but scientific studies of flatworms began only in the 18th century. It was Gegenbaur (1859) who placed these in a separate group and suggested the present name of the phylum.

General Characters

(1) They are dorso-ventrally flattened like a leaf.

(2) They show organ grade of organization.

(3) They are acoelomate animals. The cavity in platyhelminthes is filled with mesenchyme or parenchyma.

(4) They are triploblastic animals. The cells of the body wall are arranged in three layers.They are the ectoderm, the mesoderm and the endoderm.

(5) They are bilaterally symmetrical animals. The body of the animal can be divided into two equal similar halves through only one plan. Animals with this symmetry have definite polarity of anterior and posterior ends.

(6) Some members have segmented body. The segmentation in platyhelminthes is called pseudometamerism.

(7) Many of the parenchyma cells give rise to muscle fibres.The muscle fibres are arranged in circular, longitudinal and vertical layers.

(8) The digestive system is completely absent from Cestoda and Acoela. The alimentary canal is branched in Turbellarians. The anus is absent from them.

(9) The respiratory organs are absent. In parasites respiration is anaerobic.

(10) There is no circulatory system.

(11) The excretory system is formed of protonephridia (flame cells or solenocytes).

(12) Anus is absent like coelenterates, with blind sac body plan.

(13) The nervous system is well developed. It is formed of longitudinal nerve cords with ganglia. A pair of anterior ganglia form the brain. The longitudinal nerve cords are connected together by transverse connectives.

(14) They are hermaphrodites, i.e., both male and female reproductive organs are present in the same animal.

(15) Fertilization is internal in them. Self or cross fertilization takes place in them.

(16) Their development is direct or indirect. Endoparasites show usually indirect development with many larval stages. Their life cycle is completed in one or two hosts.

(17) They are free living or parasitic. In parasitic worms adhesive organs like hooks, spines, suckers and adhesive secretions are present.

Classification of platyhelminthes : On the basis of digestive tract and free living or parasitic nature phylum platyhelminthes has been divided into three classes –

Class 1. Turbellaria (L. turbella, a string)

(1) Most of the turbellarians are free living but some of them are ectocommensal or parasitic, commonly called planarians or flat worms.

(2) The body epidermis is either cellular or syncytial and covered with cilia. Epidermis contains rhabdites.

(3) Segmentation is absent.

(4) Digestive system is present except in a few.

(5) Suckers are absent.

(6) Life cycle is simple, development direct.

Example : Dugesia, Notoplana, Bipalium, Thysanozoon, etc.

• Bipalium is only terrestrial planarian.

Class 2. Trematoda (Gr. trema, hole)

(1) Ecto or endoparasites of vertebrates; commonly called flukes.

(2) Body mostly oval, unsegmented.

(3) Body wall without cilia, but covered by a thick, resistant, syncytial tegument.

(4) Suckers, and often hooks and spines, present for attachment to host tissues.

(5) Sense organs usually absent in adults.

(6) Digestive system well developed with terminal mouth, but no anus.

(7) Mostly hermaphrodite. Life cycle simple or complicated.

Examples : Polystomum, Fasciola, Schistosoma (blood fluke of man and other mammals), Opisthorchis, etc.

• Opisthorchis sinensis is commonly known as chinese liver fluke of man.

Class 3. Cestoda

(1) All endoparasites. Mostly in alimentary canal of vertebrates; commonly called tapeworms.

(2) Body long and slender, tape-like, usually divided into small segments (= proglottids).

(3) Body wall non-ciliated, with a thick tegument.

(4) Anterior end with suckers and other attachment organs.

(5) No mouth, digestive system absent, digested liquid food is absorbed from host tissues by diffusion through body wall.

(6) Sense organs absent.

(7) Each proglottid contains one or two complete sets of hermaphrodite (bisexual) reproductive organs.

(8) Life-cycle usually complicated with alternation of hosts. Embryo hooked.

Examples – Taenia, Echinococcus, Hymenolepsis, Diphyllobothrium, Echinococcus, Dipylidium.

• Hymenolepsis is dwarf tapeworm. It is monogenetic tapeworm of man.
• Dipylidium is dog tapeworm.
• Diphyllobothrium is the largest tapeworms.

• Echinococcus is also called hydatid worm. Its hydatid cyst shows exogenous as well as endogenous budding. Parasite of small intestine of dogs, cats, etc. It has only 3-4 proglottids.