Introduction to the Platyhelminthes
Brief Taxonomy of Platyhelminthes
Basic Body Plan
The Tegument
Reproductive System
Digestive System
General Lifecycle
Platyhelminthes - The Basic Body Plan
1)The Overall Organisation of the Platyhelminth Organism
L m = Longitudinal Muscle Layer
C m = Circular Muscle Layer
D m = Dorsoventral Muscle Layer
M = Mesoderm
G = Gut
f c = Flame cells
N = Nerve Cords
V = Vitellaria
O = Oviduct
S d = Sperm Duct
T = Testes
The body plan of all platyhelminthes show features that are common throughout the group. The body of the organism is usually dorso-ventrally flattened, giving the group its common name, the flat worms. The outer surface of the organism is covered by an epidermal layer, formed of ectodermal tissue, that in many groups, particularly the parasitic organisms, forms a syncytial tegument. The details of this structure is covered in a seperate section of this page.
Beneath the epidermis there are two layers of smooth muscle fibres (circular and longitudinal muscles, embedded within the mesoderm. In addition, within the mesoderm other smooth muscle fibres are arranged dorsoventraly, and the other internal organs are found embedded. The presence of the mesoderm marks an important transition from the very simple two layered animals such as sponges and cnidarians, to that of other more complex organisms.
In these simpler organisms a layer of non-cellular mesoglea is secreted between the outer ectoderm and the inner endoderm (the endoderm forming the gut). Within this mesoglea amoeboid mesenchymal cells may be scattered, and it is these that probably gave rise to the mesodermal, (or parenchymal) cells of higher organisms, from which the muscles and internal organs arise.
In the flatworms the mesoderm consists of a loose network of irregularly shaped cells, giving bulk to the organisms and supporting the internal organisms. The platyhelminthes are however still very "primitive organisms", and there is no body cavity as seen in "higher" organisms such as the coelome or pseudocoelome.
The details of the organisation of the platyhelminth sexual organs are detailed in another section of this page, but the organisation of the nervous and excretory systems will be briefly described here.
2)The Platyhelminth Excretory System
M = Mesoderm
F = Flagella
P N T = Protonephridial Tubule
The platyhelminth excretory system consists of a network of fine protonephridial tubules embedded within the mesoderm. Branching off of this network are the flame cells. These are blind ended enlargments of the tubules within which beat flagella, moving fluid within the protonephridia. Within the free living turbellarians, and the ectoparasitic platyhelminthes, this excretory systems main function appears to be to remove excess water from the body of the organism, although waste metabolic products may also be flushed out at the same time. The function this system within endoparasitic groups such as the Digeneans or Cestodes is less clear, and there are some differences in the systems organisation within these groups. Within the Digenean trematodes typically the protonephridia drain into a pair of longitudinal ducts, opening into a posterior bladder which is connected to the exterior via an excretory pore. In the cestodes the protonephridia drain into two dorsolateral ducts (interconnecting in the scolex region of the parasites body) and two dorsoventral ducts which are connected by transverse canals at the posterior of the proglottids. These ducts open to the exterior from the tapeworms termial proglottid.
3)The Platyhelminth Nervous System
B = "Brain"
N = Nerve Cords
Whilst the more "primitive" free living platyhelminthes such as the polyclads have only a simple nerve net, others have much more complex systems. In the free living turbellarians there is a mass of nervous tissue at the anterior end of the organism (the "Brain"), from which two nerve cords run down the length of the body, connected together by many transverse nerve fibres, giving a characteristic "ladder" type of construction. In addition there are numerous side branches from the two main nerve cords, many of which, in the free living groups, are connected to sensory organelles. These sensory organelles include eye spots, and numerous specialised cells, capable of detecting touch, water movement, and chemical stimuli (important in location of food). In the parasitic Trematodes and Cestodes the presence of these sensory organelles is usually much reduced, although they may be present at particular stages in the lifecycles of these organisms. For example the free swimming cercaria and miracidia use highly specialised sensory organelles to locate their hosts. The organisation of the nervous system of the adult trematode is very similar to that of the turbellarians. In the cestodes an anterior nerve mass is found in the scolex, from which two, and sometimes more, nerve cords extend throughout the length of the strobila, often conected together by nerve rings within each individual proglottid.