biology

Planarian

Planarian is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Planarian rather than just read about it. In short: A planarian (also known as a triclad; plural planarians or planaria) is a free-living flatworm of the order Tricladida. Planarians occur in marine, freshwater, and terrestrial environments, and the order is divided into the suborders Maricola, Cavernicola, and Continenticola.

Key takeaways

Planarian belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Planarian to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Planarian from memory before moving on to harder problems.

Reference excerpt

A planarian (also known as a triclad; plural planarians or planaria) is a free-living flatworm of the order Tricladida. Planarians occur in marine, freshwater, and terrestrial environments, and the order is divided into the suborders Maricola, Cavernicola, and Continenticola. Planarians are bilaterian flatworms that lack a fluid-filled body cavity and have a three-branched intestine with one anterior and two posterior branches. They lack circulatory and respiratory systems, absorbing oxygen through the body wall, and move using cilia and muscular contractions. Depending on the species, planarians reproduce sexually, asexually through fission, or by both methods. Planarians are best known for their ability to regenerate missing tissues and organs, a process enabled by abundant adult stem cells called neoblasts. Neoblasts can self-renew and generate the cell types needed for regeneration and tissue maintenance. Because of these regenerative abilities and their relatively simple anatomy, planarians are widely used as model organisms in studies of regeneration, stem cells, aging, toxicology, genetics, and memory.

Phylogeny and taxonomy

Phylogeny Phylogenetic supertree after Sluys et al., 2009:

Taxonomy

Linnaean ranks after Sluys et al., 2009:

Order Tricladida Suborder Maricola Superfamily Cercyroidea Family Centrovarioplanidae Family Cercyridae Family Meixnerididae Superfamily Bdellouroidea Family Uteriporidae Family Bdellouridae Superfamily Procerodoidea Family Procerodidae Suborder Cavernicola Family Dimarcusidae Suborder Continenticola Superfamily Planarioidea Family Planariidae Family Dendrocoelidae Family Kenkiidae Superfamily Geoplanoidea Family Dugesiidae Family Geoplanidae

Anatomy and physiology Planarians are bilaterian flatworms that lack a fluid-filled body cavity, and the space between their organ systems is filled with parenchyma. Planarians lack a circulatory system, and absorb oxygen through their body wall. They uptake food to their gut using a muscular pharynx, and nutrients diffuse to internal tissues. A three-branched intestine runs across almost the entire body, and includes a single anterior and two posterior branches. The planarian intestine is a blind sac, having no exit cavity, and therefore planarians uptake food and egest waste through the same orifice, located near the middle of the ventral body surface.

The excretory system is made of many tubes with many flame cells and excretory pores on them. Also, flame cells remove unwanted liquids from the body by passing them through ducts which lead to excretory pores, where waste is released on the dorsal surface of the planarian. The triclads have an anterior end or head where sense organs, such as eyes and chemoreceptors, are usually found. Some species have auricles that protrude from the margins of the head. The auricles can contain chemical and mechanical sensory receptors. The number of eyes in the triclads is variable depending on the species. While many species have two eyes (e.g. Dugesia or Microplana), others have many more distributed along the body (e.g. most Geoplaninae). Sometimes, those species with two eyes may present smaller accessory or supernumerary eyes. The subterranean triclads are often eyeless or blind. The body of the triclads is covered by a ciliated epidermis that contains rhabdites. Between the epidermis and the gastrodermis there is a parenchymatous tissue or mesenchyme.

Nervous system

The planarian nervous systems consists of a bilobed shaped cerebral ganglion that is referred to as the planarian brain. Longitudinal ventral nerve chords extend from the brain to the tail. Transverse nerves, commissure, connect the ventral nerve chords forming ladder-like nerve system. The brain has been shown to exhibit spontaneous electrophysiological oscillations, similar to the electroencephalographic (EEG) activity of other animals. The Planarian has a soft, flat, wedge-shaped body that may be black, brown, blue, gray, or white. The blunt, triangular head has two ocelli (eyepots), pigmented areas that are sensitive to light. There are two auricles (earlike projections) at the base of the head, which are sensitive to touch and the presence of certain chemicals. The mouth is located in the middle of the underside of the body, which is covered with hairlike projections (cilia). There are no circulatory or respiratory systems; oxygen enters and carbon dioxide leaves the planarian's body by diffusing through the body wall.

Reproduction

Triclads reproduce sexually and asexually, and different species may be able to reproduce by one or both modes. Planarians are hermaphrodites. In sexual reproduction, the mating generally involves mutual insemination. Thus, one of their gametes will combine with the gamete of another planarian. Each planarian transports its secretion to the other planarian, giving and receiving sperm. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. In asexual reproduction, the planarian fissions and each fragment regenerates its missing tissues, generating complete anatomy and restoring functions. Asexual reproduction, similar to regeneration following injury, requires neoblasts, adult stem cells, which proliferate and produce differentiated cells. Some researchers claim that the products derived from bisecting a planarian are similar to the products of planarian asexual reproduction; however, debates about the nature of asexual reproduction in planarians and its effect on the population are ongoing. Some species of planarian are exclusively asexual, whereas some can reproduce both sexually and asexually. In most of the cases the sexual reproduction involve two individuals; auto fecundation has been rarely reported (e.g. in Cura foremanii).

Neoblasts

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Planarian

Start with the simplest possible case. Write down what Planarian claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Planarian before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Planarian ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Planarian

In research: Planarian appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Planarian in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Planarian is common in secondary-school and first-year university syllabi. It links to neighbouring topics Animal models, Invertebrate common names, Negligibly senescent organisms, so understanding it makes those chapters shorter.
In everyday life: Look for Planarian outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Planarian in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Planarian means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Planarian out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Planarian in simple terms?

Why does Planarian matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Planarian?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Planarian.