Planaria (Platyhelminthes)

external image I10-82-planarian.jpg
This image shows the various different systems of the Planaria

Classification/Diagnostic characteristics:
  • thin bodies
  • Platyhelminthes means flat worm
  • Range in size from microscopic to over 20m long
  • Planaria are in the class called Turbellaria
  • can be black, gray, white or brown
  • has ocelli, areas that respond to light
  • has auricles which respond to touch and chemicals.
The planaria has an acoelomate body, which means it has no internal cavity to hold organs, no anus, and lacks a circulatory system.
Relationship to Humans:

Planaria are used often as subjects in laboratory experiments. Specifically, planaria are useful in studies of regeneration, leaning and behavior. By analyzing how the planaria responds to stimuli, scientists study the process of memory and memory formations in learned responses. Link:
Predator avoidance:
Planaria have developed anti-predator behavior in the form of avoidance behaviors that result from chemical cue signals. This is an example of a behavioral response that is highly functional in group settings. The avoidance behavior is triggered by self-released chemical signals within an injured planaria that triggers the release of chemicals into the surrounding environment. Other planaria, and even some other species, are able to react to these water-borne chemical signals by triggering their own avoidance behaviors and (hopefully) avoid the initial threat. The chemical reaction had no benefit to the individual, but acts as a warning to the population and therefore conveys advantage to the population as a whole. (2)
Nutrient acquisition:
  • Carniveores
  • Feed on smaller or dead animal
  • feed on liver of other animals

Planarians are usually either carnivorous or scavengers. As carnivores, they may feed upon small worm crustaceans, snails, or dead animal debris. The mouth is located near the middle of the ventral surface. To eat, a tube-like pharynx can be extended from the mouth and inserted into the prey. Through this tube, it secretes digestive juice onto its prey, then sucks in bits of partly-digested food externally before it is sucked into the gut. So, it uses both extra and intra cellular digestion. During extra-cellular digestion, food enters from the pharynx into 3 intestinal branches (one anterior and two lateral) and sub branches. Each sub-branch's end is sac-like. Digestion will occur in the intestine. During intra-cellular digestion, the semi-digested food enter the cells for complete digestion. Sometimes, food may be absorbed by diffusion.
Taken from the website:

Reproduction and life cycle:
  • Reproduce asexually through regeneration. The parent constricts in the middle, and each half regenerates the missing end.
  • Sexual reproduction can also occur.
  • Copulating mates cross-fertilize.

Planaria are hemaphrodites, as they possess both testicles and ovaries. Thus, they sexually reproduce by combining their gametes to form another planarian. Planaria are also capable of asexually reproducing; the planarian will detach its tail in half and regrow the lost part through regeneration, allowing neoblasts (adult stem cells) to divide and differentiate. There are some problems with asexual reproduction, however, so planarians tend to sexually reproduce with other planarians, enhancing survivability. (BHu)

Growth and Development:
  • will pull itself in half-tail will regernate a new head and head will regenerate a new tailplanaria-regeneration.jpg
  • In addition, unintended cutting of the planaria will still result in full regeneration from even small pieces.
  • Often parasitic they will go trough a number of hosts in their lifetimes. When they return to the intial host in which they were born, they will die and a new generation will begin.external image image025.jpg


  • Composed of a single layer of ciliated epithelium (lining)
  • Between the cilia there are often long flagella and stiff hairs and sometimes even tiny spines

  • Move by using cilia on the ventral epidermis
  • Glide along mucus that they secrete
    While all planaria use secreted mucous to assist in movement, as stated above small planaria use cilia for movement while larger planaria move by way of muscle contractions.-

Sensing the environment:

Planaria have ocelli (eyespots) with light-sensitive pigmented areas. They also have two auricles, which contain mechanoreceptors and chemoreceptors at the base of the head. The planarian has a cerebral ganglion, sometimes referred to as a brain, which is connected to a ladder-like system of nerves which run through the body, allowing for effective coordinated muscle responses to stimuli. (

Planaria possess a rudimentary brain consisting of two ganglions, or groups of neurons, located at the anterior end of the organism. Two nerve cords run along the side towards the back of the animal, giving the nervous system a ladder-like appearance and allowing for the ability of displaying varied behaviors. The concentration of all sensory organs at one end of the body is called cephilization. Planaria are capable of responding to their environment by moving towards or away from a stimulus, with positive responses and negative responses being collectively known as taxis. The many different types of taxis include chemotaxis (chemicals), phototaxis (light), and mechanotaxis (physical touch), all of which are detected using different sensory receptors. Providing the organism with the ability to move through its environment in order to approach food sources, avoid bright light, and avert overly hot areas, the little flagellum at the posterior end spins in response to certain signals and functions as a motor that propels the organism in the right direction. Planarians exhibit three observable forms of taxis: negative phototaxis, or moving away from light; positive chemotaxis, or moving towards food; and negative mechanotaxis, or moving away when touched. Moreover, the planarian is equipped with a simple sensory system, with two eyespots being present on the head of the animal. The eyespots contain photoreceptors that allow the planarian to detect the presence of light, as opposed to the human visual system, which allows for the processing of complex visual information. The information from the eyespots is transferred to the neural ganglia, where it is processed and subsequently causes the motor system to be activated. (Alexander Soloviev)

Gas Exchange:
  • Lack organs for gas exchange and circulation
  • The flat shape of the body places all cells close to the surrounding water, and fine branching of the gastrovascular cavity distributes food throughout the Planaria.
  • external image 315801-Platyhelminthes_Planaria_2.gif
Waste Removal:
  • Nitrogen waste in the form of ammonia diffuses directly from the cells into the surrounding water.
  • Simple excretory apparatus that functions to maintain osmotic balance between the Planaria and its surroundings. Ciliated cells called flame cells waft fluid though the branched ducts opening to the outside.
Environmental physiology:
Interestingly, planaria are able to be kept in a refrigerator and still be living and they also reproduce by regeneration so if they lose a body part they can easily grow another part back. (evan). (
Internal Circulation:
-lacks a circulatory system
Chemical control:


1. Explain the process of food acquisition in the Planaria and then how the food is processed and removed? (Mention some cells used in this process.)

2. Explain the three types of taxis that Planaria exhibit when responding to environmental stimuli. Be sure to explain what a taxi is.

2. Planaria are relatively simple organisms, but they are able to respond to and distinguish between different stimuli. What mechanisms do the planaria use to detect these stimuli? Be sure to integrate your knowledge of the planaria's nervous system in your answer.

4. How are planaria dependent on one another for survival?