Platyhelminthes

Members of this Phylum are known as “Flatworms” as they are flattened dorso-ventrally and are bilaterally symmetrical. All possess a definitive head or head end with mouth pore or without mouth pore. Organs of attachment are suckers which may be armed with hooks. They are usually leaf-shaped or oval but some are very elongate. A coelom is absent and the organs are embedded in tissue called parenchyma. The digestive system is incomplete or absent and the functional unite of excretory system is represented by ‘Flame cell’ or ‘Protonephridium’. The nervous system consists of a pair of anterior ganglia with one to three pairs of longitudinal nerve cords connected by transverse commissures. Most species are monoecious (bisexual) or (hermaphrodite) but a few are dioecious (unisexual). All Platyhelminths lack skeletal, circulatory or respiratory structures, and adults lack locomotory organs. Life cycle is direct or indirect (involving an intermediate host).

The phylum Platyhelminthes includes three classes such as:

1. Class: Turbellaria

Most turbellaria are free-living in salt and fresh water. Some are parasitic on or in echinoderms, crustaceans, moulluscs annelids, arachnids and coelenterates. Most turbellarians possess a small pharynx, a digestive tract that include short diverticula, a paried protonephridia and excretory pore. There is one pair of ovaries and from two to many testes. The nervous system consists of a cephalic ganglion ituated near the pharynx giving rise to several nerve fibers. Many species possess a pair of eye-spots. They have a ciliated epidermis and a undivided body and a simple direct life cycle.

2. Class: Trematoda (Flukes)

All trematodes or flukes are either endo or ecto-parasitic. Ectoparasitic trematodes develop directly in or on a single type of host and are said to be monogenetic, endo-parasitic trematodes develops through a sequence of young individuals unlike the parent with one or more change of host and are known as digenetic.

Most trematodes are pale cream in color, but some may be slightly reddish due to engorgement or blood. Most trematodes are less than 30 mm in length; but some biggest species like Fasciola gigantica, 75 mm long and smaller like Heterophyes heterophes 2mm long may also be found. The body is usually covered by a smooth tegument (cuticle), but some times the cuticle may be spiny or scaly. The mouth is situated forward and at the tip of the body, the digestive systems is well developed and the intestine is bifurcated. Trematodes cling to the body of  host or remain attached to organs by means of organs of adhesion known as suckers. Suckers may sometimes be armed with hooks or spines. excretory pore is single in most case opening posteriorly and paired spending anterodorsally.   

3. Class : Cestoda (Tapeworms):

The word tapeworm has been derived from the german word “Bundworm” meaning “ribbon worm”. The term cestoda has originated from Greek word ribbon and this refers to the body of the worm which is long flattened and cylindrical. A tapeworm is usually white but occasionally is gray, yellow, or cream in colour depending on the absorption of substances in its environment. The surface of the body is rarely smooth but is commonly cross wrinkled or grooved longitudinally. The cuticle is permeable to nutrients. Like other flatworms the tapeworms lack a body cavity, organs being suspended in sponge like tissue the parenchyma. The anterior end of the tapeworm is small, spherical and is designated as a holdfast organ commonly but falsely known as head. The holdfast organ is commonly accepted as ‘Scolex’. Following the scolex is a small unsegmented portion known as ‘neck’ or ‘growth zone’ from where the segments or proglottids, are budded off. The rest of the body is referred as the ‘Storobila’ consisting of segments or proglottids. The acetabulate scolex has four muscular cups, suckers sunk in to the surface. These suckers are equidistant and the equitorial region of the scolex; the rim of each is round, oval or occasionally slitlike. Commonly they apical end of the hold fast region projects like dome or finger and is called “rostellum” and the rostellum usually possess hooks or spines. The bothriate type of scolex has two depressions which are known as “bothria”. In bothridial type scolex has four “bothridia” which are outgrowths from the surface and are like as in Phyllobothride. With the help of the holdfast structures the tapeworms can also move to a preferable site.

Osmoregulatory excretory system consists typically of four canals which run the full length of the worm and lie just inside the margin of the medullary parenchyma. The flame cells occur in groups and they lie the parenchyma in the vicinity of the ventral vessels.

The nervous system consists of four ganglionoid structures, lie in pairs in the scolex, two on the right side and two on the left side. These gives of branches anteriorly and posteriorly, joins in paris and the trunks run through the strobila. The main posterior trunsk are lateral to the ventral vessels.

Its reproductive system, is complex, in most cases each proglottid is bisexual, some being doubel bisexual.