General Characteristics of Platyhelminthes:
Platyhelminthes are triploblastic and bilateral in proportion. They are not divided. They are aceolomate, flat in shape and contain only 1 opening, mouth without the anus.
Triploblastic Condition of Platyhelminthes:
In platyhelminthes, embryo splits in 3 instead of 2 as it does for cnidarians. With this extra layer of cells, called mesoderm, the platyhelminthes:
i) Contain the increase in size
ii) There is then separation of alimentary canal from the body wall
iii) The diversity of organs are now formed
iv) There is movement towards organ-system level of organization.
v) With it muscles are starting to develop. Cilia and flagella are turning out to be inappropriate for larger sizes evolving.
The Acoelomate Condition:
When animals are larger, problems of how to transport expensive life supporting materials to every part arise. As plants grew to the huge sizes, vascular tissues that serve for transportation had to develop. Here with emergence of mesoderm, problem of how to get materials from ectoderm to endoderm and vice versa has occurred. In platyhelminthes mesoderm entirely fills space between ectoderm and endoderm.
This filling of whole space between ectoderm and endoderm with (by) mesoderm is acoelomate condition. Animals in this group don't have the choice but be flat to present the large surface area for dispersion to satisfy metabolic needs. This is why they are known as flatworms. In other animals, space grows inside mesoderm. This space is known coelom. Such animals will thus be explained as coelomates.
Platyhelminthes are first group of animals to develop organ - system level of organization from mesoderm. Though, many mesoderms stayed undifferentiated into cells and tissues. Mesoderm here just forms the packing tissue known as mesonchyme. It supports and defends body organs.
Classification of Flatworms:
1) Class Turbellaria:
The Turbellaria are free living or commensal with bigger animals. There are approx 3,000 known species of Turbellaria, most of which are marine. They are most ancient of Platyhelminthes, and other three classes of Platyhelminthes all developed from the Turbellarians. Tubellarians are the single free living class in the phylum. Planaria, a tubellarian, is free living through carnivorous. It is discovered in fresh water streams and ponds. It performs just at night to feed and hides under stones during day. Its body is soft and black. It is flat, wider towards head end and narrower at other. It can estimate up to 15mm long. It has the pair of eyes at top of head. There is also the mouth at under part of body, at end, opposite to head. It is bilaterally proportioned. It feeds on small crustaceans, worms and dead bodies of larger animals.
2) Class Monogenea:
The class monogenea is differentiated by most of members being, while Digeneans and Cestodes are all endoparasites. To facilitate their parasitic life style Monogeneans have complicated attachment organs at posterior or tail end of their bodies, frequently having the mixture of suckers, clamps, hooks and spines.
3) Class Cestoda:
Cestodes or tapeworms are most specialised of the Platyhelminthe parasites. All cestodes contain at least one, and at times more than 1, secondary or intermediate host and their primary host. While intermediate hosts are frequently invertebrates of some kind, primary host is usually a vertebrate.
4) Class Trematoda:
The Aspidogastrea are the interesting group of approx 80 species of parasitic Platyhelminths. They are all aquatic and have indirect life cycles, like they have more than 1 host species. Many species use some kind of mollusc or arthropod as intermediate host and vertebrate like fish or turtle as primary host. Fasciola is the example in this class. It is the endoparasite. It lives in bile duct of its main host that could be sheep, cattle or even man. As the endoparasite, it has characteristics that are extremely dissimilar from free living planaria. It contains oral and ventral suckers with which it sticks to its hosts. It has the complicated life history that involves 3 larval stages. These give opportunity for it to rise in number significantly. These large numbers are essential to offset loses which should essentially happen as it attempts to locate new hosts. Snail is its secondary host. In every stage, animal grows aspects with which it suited to living in the host.
It is flat and thin, like the leaf. It has gland cells in the body wall that secrete materials in opposition to host's antitoxins. Every liver fluke is male and female put together. It redevelops by self or cross fertilization. It can stay alive anaerobically if there is no oxygen. Body is covered with thick spiny cuticle for protection. Planaria didn't have such protective features. These are significant for fasciola due to its endoparasitic form of life. The outer surface of free living is tubellarians. Tubellarians include cilia for moving to get the food. The adult fasciola contains sensory organ. Only free living larval stages have.
The example is Taenia (the tape worm). It is endoparasitic, flat and long and body is separated into proglottides which can break off. Every proglottide is fundamentally made up of reproductive organs to makes sure that it redevelops in large numbers and continues to carry out itself in host. It also contains strong suckers and hooks on the head with which it fastens itself to host. Body has thick cuticle for protection. It has no cilia. As it doesn't require digesting the food and lives within the host most part of life history, it doesn't have the digestive system or sensory organs. Only free living larval stages include. It though has nerve ganglia at head that sends down nerve cord down its sides. There are as well excretory canals beside its sides.
Life Cycle of Tape Worm:
As stated earlier, every proglottide is packed with sexual structures. Opening of female system is located close to cirrus which is male organ. This cirrus initiates sperm into vagina. Sperms are performed to sperm to fertilize eggs. Fertilized grow and fills proglottide that can now be separate and comes out of host's gut with faeces. These are then taken up by secondary (intermediate) hosts like cow and pig in their own feeding. Once in host, embryo inside egg is liberated. This embryo is previously prepared with hooks by that it bores its way through wall of intestine of the host unto blood system. Blood now carries it round body to connective tissues of muscles where it settles and becomes the bladder or cyst full of fluid. This cyst also has the head with 4 suckers on wall. The head is at first in stage turned inside out. Cattle or pig so infected carries the bladder worm in tongue, tissues, heart and jaw. If they are now eaten by man (final host), head of bladder worm turns out and with suckers, it sticks itself into wall of gut and cycle starts again.
Life cycle of Taenia:
Structurally, flat worms are flat and soft. The 2 parasitic classes have cuticles on their body to shield them from enzymes of the host. Free-living tubellarians don't have such protection. Due to their parasitic modes, cestodes and tremode have devices by which they join themselves to the hosts. Free living tubellarians have the good regeneration power which parasites do not appear to have. They also reproduce profusely to make sure continuity. Sense organs aren't discovered in encoparasitic classes. They only exist in free living phases in life cycles. Instead the parasitic forms branched internal organs to ease competent absorption and excretion in the host.
3) The significance of Platyhelminthes
Phylum is very significant to man as their disease causing groups that infect both man and livestock. They cause loss of the lot of financial resource in treatment of the infection. They also cause pain and disability to man.
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