Flatworms have three embryonic tissue layers that give rise to surfaces that cover tissues from ectoderm , internal tissues from mesoderm , and line the digestive system from endoderm.
The epidermal tissue is a single layer cells or a layer of fused cells syncytium that covers a layer of circular muscle above a layer of longitudinal muscle. The mesodermal tissues include mesenchymal cells that contain collagen and support secretory cells that secrete mucus and other materials at the surface. The flatworms are acoelomates, so their bodies are solid between the outer surface and the cavity of the digestive system. The free-living species of flatworms are predators or scavengers.
Parasitic forms feed on the tissues of their hosts. Most flatworms, such as the planarian shown in Figure 1, have a gastrovascular cavity rather than a complete digestive system. Some species also have an anal opening. The gut may be a simple sac or highly branched. Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis. One group, the cestodes, lacks a digestive system.
Flatworms have an excretory system with a network of tubules throughout the body with openings to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body.
The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerves at the anterior end of the worm, where there may also be a concentration of photosensory and chemosensory cells.
Figure 1. The planarian is a flatworm that has a gastrovascular cavity with one opening that serves as both mouth and anus. The excretory system is made up of tubules connected to excretory pores on both sides of the body. The nervous system is composed of two interconnected nerve cords running the length of the body, with cerebral ganglia and eyespots at the anterior end. There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on diffusion and cell-cell junctions.
Most flatworm species are monoecious, and fertilization is typically internal. Asexual reproduction is common in some groups. Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda Figure 2. As discussed above, the relationships among members of these classes is being reassessed, with the turbellarians in particular now viewed as a paraphyletic group, a group that does not have a single common ancestor. Figure 2.
Phylum Platyhelminthes is divided into four classes. In their search for the answer to this question, scientists have a population of small cells in their sights, namely the approximately five-micrometre-long neoblasts.
These cells are found almost everywhere in the planarian body and behave like stem cells: they divide, renew and can form the different cell types that have been lost as a result of amputation Fig. When the planarian loses a body part or discards its tail for reproduction, the neoblasts are reactivated and migrate to the wound. They divide there and their offspring form a blastema, in which — as a result of interplay between various extra- and intra-cellular factors — important differentiation and patterning processes take place.
Thanks to these processes, in turn, complex structures like the brain are formed. If the neoblasts are eliminated through radiation, for example, the planarian loses its ability to regenerate and dies within a few weeks.
The fact that, following transplantation into an irradiated, neoblast-free worm, a single neoblast can produce all cell types and enable the host worm to regain its ability to regenerate shows that at least some neoblasts are pluripotent [2]. In healthy mammals, pluripotency, that is the ability of one cell to produce any given cell type found in an organism, e.
Therefore, stable pluripotency in the adult organism is something special but not impossible as long as mechanisms exist for conserving this characteristic — as is clearly the case with the planarians. A Planarians are able to re-grow an entire head in a matter of a few days. During regeneration, when a lot of new tissue has to be produced, they are able to generate a wide variety of cell types.
The cell nuclei are marked in blue. Tissue-specific markers are marked in red, green and white. Figure 1A adapted from [1]. The preservation of pluripotency has been an important topic in stem cell research for years, and has mostly been examined up to now using isolated embryonic stem cells. Important transcription factors that can induce and preserve pluripotency were discovered in the course of this research. So what can planarians contribute to the current research if their stem cells cannot be cultivated and reproduced outside of the body?
This is precisely where the strength of the planarians as a model system in stem cell research lies: the combination they can offer of a natural extracellular environment and pluripotent stem cells. Whereas cultivated stem cells are normally taken out of their natural environment and all important interactions with neighbouring cells and freely moving molecules are interrupted as a result, the stem cells in planarians can be observed and manipulated under normal conditions in vivo.
Although planarians have been renowned as masters of regeneration and research objects for generations, they have undergone a genuine explosion in research interest in recent years. In particular, the possibility of switching off specific genes through RNA interference RNAi and the availability of the genome sequence of Schmidtea mediterranea , a planarian species which is especially good at regenerating itself, have contributed to this surge in interest.
Hence, it is possible to examine which messenger RNAs mRNAs are produced that act as molecular templates for the production of proteins. However, the real work only starts here: the extent to which the presence of a particular mRNA also reflects the volume of protein that is active in the cell remains to be determined. It is mainly the proteins in the form of enzymes, signalling molecules and structural elements, and not their mRNAs, that ultimately control the majority of cellular processes.
In addition, their production using mRNA templates and their lifetime are precisely regulated processes and the frequency with which an mRNA arises cannot provide any information about these processes. The time has come, therefore, to develop experimental approaches for planarians that extend beyond gene expression analysis and lend greater significance to the subsequent regulatory processes.
Combined with the marking of proteins, quantitative mass spectrometry, which enables the identification of thousands of protein fragments based on their mass, provides a starting point here. The natural amino acid lysine, for example, contains six carbon atoms, each of which has six neutrons and six protons 12 C6 lysine.
This enables the direct comparison of the abundance of a particular protein present in marked and non-marked samples. Life Cycle Life Cycle. Human Connections Since they breathe through their skin, flatworms are sensitive to water quality and serve as indicators of reduced oxygen and other changes in their habitat.
Ecosystem Connections Ecosystems are based on the tiny plants and animals that form the base of food chains. Like many other cave-dwelling animals troglobites , the pink planarian lacks the ability to sense light. Right to Use. The rectangular end at upper left is the head, and the whitish part at that end is the brain. Pink planarians adhere to the bottoms of rocks under water. If the spaces between the rocks become clogged with dirt, their habitat is harmed.
Similar Species. Horsehair Worms. Tubificid Worms Tubifex Worms. Midge Fly Larvae. About Aquatic Invertebrates in Missouri.
Missouri's streams, lakes, and other aquatic habitats hold thousands of kinds of invertebrates — worms, freshwater mussels, snails, crayfish, insects, and other animals without backbones. These creatures are vital links in the aquatic food chain, and their presence and numbers tell us a lot about water quality. Freshwater Mussels Facts. Crayfish Facts. Aquatic Invertebrate Facts.
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