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Apparatus for the experiment


Discuss about the Invertebrate Responses to External Stimuli.

Placing ten invertebrates on a petri dish of choice determines their movement rate. Earthworms (Lumbricus terrestris) were chosen for this experimental study. For this, the earthworms were dipped in cold water kept on a petri dish. The experimental results were derived by measuring the organism response by the utilization of the qualitative data and the quantitative data. Qualitative data was determined by counting the number of living worms at the end of the study and quantitative data was determined by description of the conditions for the reaction of the worms towards the applied stimuli. The study was further repeated by the application of various conditions as for the measurement of the response to the stimuli and the applied conditions for stimuli were room temperature, hot water, a combination of light and room temperature and yeast suspension.

The primary purpose of the experimental study was the observation of the invertebrates’ stimuli in the various environments. It is a natural phenomenon that animals keep on adjusting their environment with the required changes according to the environmental changes that occur around them. This is because of the fact that they possess special sense organs that help them to response according to the stimuli of the changing environment. These changes are often regarded as mutuality relationships that exist between the animal and its surrounding atmosphere. These stimulus based adaptive responses are essential for the survival of the animal species.

The purpose of this experimental study was the investigation of the invertebrate responses towards external stimuli that consist of hot water, light, water at room temperature, yeast suspension and cold water. The responses of the worms in different atmospheres or external stimuli were different. Like, when dipped in cold water, their rate of movement increases with few ending up in curling and when dipped in hot water, the movement rate was extreme and resulted in death. Apart from these, the other applied external stimuli were water at room temperature and under light in the room temperature water. In both the cases, the movement rate was fast with no death. In the yeast suspension, the rate of movement was quite fast with death resulting in few of them.

The result of the present experimental study is expressed in terms of both qualitative and quantitative data. Qualitative data for determining the number of living worms and quantitative data for describing the reactions of the earthworms towards the applied external stimuli.

Definition of external stimulus is an essential aspect for understanding this experiment. Stimulus is a phenomenon that incites exertion or action and quickens the rate of action. Invertebrates are defined as the animals that lack internal skeleton and do not possess backbone. Earthworms are the best example for this class of animals.

While performing the experiment, gloves should be worn while handling the animals for protection against contamination. The hands should be thoroughly washed with soap before and after handling the invertebrates.

Materials for the experiment

The worms should be handled with care and they should be treated gently and with proper respect. After completion of the experiment, the worms should be placed back in the beaker kept on the table for the teacher.

Apparatus for the experiment

Lamp with reflector

Beaker – 200 mL



Petri Dish




Materials for the experiment



Hot water

Ice cubes and Water

Tap water


For the study, an invertebrate animal was chosen, like, an earthworm. The experimental design was based on the background and necessary information about the invertebrate species.

A relevant stimulus was selected for producing the consistent invertebrate response like hot water, cold water, yeast suspension, light and water at room temperature.

A hypothesis was formulated for recording the response of the invertebrate to the external stimuli.

The design of the experiment was based on the negative or positive response of the organism to the selected stimulus. The experimental design had one variable independent with the other variables kept constant. The necessary controls were exerted with the experimental apparatus.

The choice chamber was designed to keep one of its sides cold with ice cubes and the other side at room temperature. The experiment was so designed that it can be differentiated between the side of attraction and repulsion of the organism.

The organism response was the dependent variable was measured in the study by the qualitative (descriptive) data and quantitative (numerical) data.

For achieving reproducible and reliable results, the experiment was repeated twice.

Data table was created for recording the observations.

The teacher approved the plan after reviewing it.

The experimental study was performed.

Petri Plate

External Stimuli

Response to Stimuli


Cold water

Rapid rate of movement, curling and death


Hot water

Extremely rapid rate of movement and death


Room temperature

Moving and alive


Yeast suspension

Quite rapid rate of movement and alive


Light and water at room temperature

Rapid rate of movement and alive


The results collected from the experimental study were presented in a tabulated format. Based on the results, it can be said that the worms placed on the various petri dishes under different external conditions of stimuli exhibited different responses. In the first petri dish, cold water was present in which 10 worms were placed. The reaction to this action was that the worms started to move fast, curled up and eventually died. In the second petri dish, warm water was placed and when the worms were placed in it, they started to move very fast and instantly died. In the third petri dish, water at room temperature was kept and the worms moved fast in it and all were alive. In the fourth and fifth petri dish, there was yeast suspension and water at room temperature under light where all the worms were alive and they moved quite rapidly for both the cases.

The results of the study demonstrated that there was significant response from the animal to the change in external stimuli and their behavior is overt. In case of invertebrates, they respond to the applied external stimuli and the corresponding activities that are correlated with the internal needs of the organism (John R Meyer, 2006). Different external environments in the petri plates demonstrated different movement types which are called as orientation behaviors. Coordinated movements constitute the orientation behaviors that includes swimming, flying, walking along with many activities that happen as an external stimuli response (John R Meyer, 2006). These are the adaptive survival values that help the invertebrate for locating the stimulus source.

Few ambiguities were noticed in the laboratory during the experiment. Worms were used as the primary assumption source for studying the invertebrates’ movements in response to a stimulus. Not every invertebrate exhibits similar response like a worm and therefore the validity of the study is under question. The living worms indicated a negative or positive response to the external stimuli. The assumption that all the worms were alive when they were introduced to the petriplates is quite doubtful as they were in clumps inside the storage bottle. Therefore, calculating the number of alive or dead worms after the experiment is not an accurate procedure for the study.

The desired response can only be achieved if the surrounding temperature is neither too hot nor too cold during the experiment. Surrounding environment plays a crucial role regarding the invertebrates’ movements, especially for a worm. Before the commencement of the experiment, if the worms are placed in extreme conditions of temperature, their movement might change. Their movements on the petriplates may or may not be exclusively due to the applied stimuli and it has the impact of the external environment as well. Prior to the experiment, the worms are soaked in various types of solutions. This is not a very accurate practice as it has not been pre-determined the attraction or repulsion of the worm for the different solutions as external stimuli factors. Different types of worms must be used for the study, as this will justify the response of different invertebrates to similar or different external stimuli.

Safety precautions must be observed during the experiment like wearing glasses and safety goggles, gloves and aprons as the study involve working in proximity to the lights bulbs that are exposed and have chances of shattering. Since the worms are collected from soil, they have stains on them and therefore apron and gloves can prevent shirts and hands from getting dirty. Hot water should be handled carefully as this may cause burns and boils while pouring it on the petri dish.


The response of the invertebrates to the applied external stimuli is determined by the number of worms that were dead or alive and from the rate of movement of the earthworms on exposing them to the different solutions contained in the petri dish, each representing different external stimuli. The number of worms that remained alive, after the experimental study, indicated the adaptation of the worm to particular external stimuli.


Lener Walter, ( 2009). Responses to Stimuli: The Basis of Behavior. Retrieved from

Nuffield Foundation, Org. ( 2012). Chamber to Investigate Animal Responses to Stimuli. Retrieved from

Wilson W. Jeffrey, (2012). Escape and avoidance learning in the earthworm. Retrieved from

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