The phylum platyhelminthes consists of all those animals that are bilaterally symmetrical and have non-segmented bodies. These animals have flattened bodies. They do not have definite respiratory and circulatory systems. The phylum platyhelminthes derives its name from two Greek words: platy - meaning flat and helminth - meaning worm. This etymology has given the phylum the more common name of 'flatworm'.
Given below are the most basic and defining characteristics of this phylum.
Platyhelminthes live in water, land or as parasites. Their respiration is generally anaerobic. They have a complex life cycle. They move around using 'cilia', which are tiny hair on their bodies. Their diet includes small animals and insects. They sense food with the help of sensory organs situated near the head.
The animals in this phylum are bilaterally symmetrical, which means that they have a definite front and rear end. They also have definite dorsal (top) and ventral (bottom) surfaces . They have an acoelomate structure, which means that they don't have a body cavity; instead all the organs directly touch the layer of tissue cells that line the body from inside. The body of the animals is divided into 3 layers which makes them triploblastic. These three layers are the ectoderm layer, the mesoderm and the inner endoderm layer. The mesoderm contains muscular layers of cells which give internal support to the organs of the flatworm. The presence of this mesoderm is what takes these animals one step higher on the evolution ladder, from simple two layered animals.
The animals in this phylum have a nervous system, a reproductive system and an excretory system. Let's see what each of these systems is like.
- Nervous System: The nervous system is made up of nerve tissues and nerve chords. The nerve tissues are located at the front end of the organism and can be called the 'brain'. From this mass of nerve tissues, two nerve chords separate and run through the entire length of the body. From these nerve chords rise other nerve branches all across the body, much resembling distributaries separating from a river. These branches are connected to sensory organs which detect sight, touch, movement, etc.
- Reproductive System: Flatworms are capable of sexual reproduction. However, they are also good examples of asexual reproduction in animals. Asexual reproduction is seen in the more primitive forms of the organism, while sexual reproduction is more evident in the complex forms. Asexual reproduction is carried out through fission and for sexual reproduction (though most platyhelminthes are hermaphrodites, self-fertilization does not take place), the penis comes out of its genital pore and fertilizes the ovary of the female, thus forming the egg and its yolk. The distinct feature about the yolk of these organisms is that it is formed outside the egg, surrounding it. Fertilization takes place and the egg moves down from the ovary duct and finally out of the genital pore.
- Excretory System: Though platyhelminthes do not have an anus for excretion, they do have excretory pores. In the excretory system, there are flame cells (enlarged cells) which have tube like structures called protonephridia. These tubes collect excess water and other waste material from within the body and excrete it through pores called nephridiopores.
The phylum platyhelminthes can be divided into four major sections: Turbelaria, Trematoda, Cestoda and Monogenea. Given below is a list of examples with its sub divisions.
- Notoplana acticola
- Dugesia tigrina (water planaria)
- Oyster leech
- Bipalium pennsylvanicum (land planaria)
- Fasciola hepatica
- Taenia Solium
- Taenia segineta
- Taenia Saginata
- Hymenolepis nana
- Oculotrema hippopotami
- Entobdella soleae
- Polystoma intergerrimum
- Gyrodactylus elegans
Hope this article has rekindled your interest in the wonderful world of biology and helped you understand this phylum better.