Cnidarian Lab: Hydra Anatomy 







     What do hydras look like? Hydras, a type of Cnidarian, are some of the simplest known animals to have a nervous system. Cnidarians gather information from their environment using specialized sensory cells. Both polyps and medusas have a nerve net, shown in the figure below. A nerve net is a loosely organized network of nerve cells that together allow Cnidarians to detect stimuli such as the touch of a foreign object. The nerve net is usually distributed uniformly throughout the body, although in some species it is concentrated around the mouth or in rings around the body. Cnidarians also have statocysts, which are groups of sensory cells that help determine the direction of gravity. Ocelli (oh-sel-eye; singular: ocellus) are eyespots made of cells that detect light; hydras lack these organs.  Many hydras, such as the green hydras, have a symbiotic relationship with algae.  The algae have a place to live and provide nutrients to the hydra by performing photosynthesis. Hydras are only two cell layers thick polyps with a thin layer of jellylike fluid between the layers. The outer layer is called the endoderm, the inner layer is called the gastroderm, and the layer in between is the mesoglea.  The body wall is so thin that respiration, circulation, and excretion of nutrients and waste can be done by simple diffusion. Tentacles with cnidocytes cells (containing stinging nematocysts) surround the mouth that also acts as the anus. Hydras are carnivorous and eat small animals such as water fleas. Their body cavity and water primarily helps to create a “hydrostatic skeleton” to allow the animal to move. Finally, hydras reproduce primarily by performing asexual reproduction by creating buds that drop off and develop into an adult in a favorable environment. The hypothesis is, if one were to observe hydras, then a better understanding of their anatomy will be made evident.  





Brown and/or Green Hydras  


Petri dish (small)  

Eye Dropper

Colored Pencils





  1. Using an eye dropper, carefully remove 1 brown or green hydra(s) from their container with minimal amount of water and place the Cnidarian (and water) on to a clean dish. Carefully take dish back to your scope (remove slide manipulator from stage of microscope if you have one)    
  2. Observe the hydra under the microscope at different levels of power starting with the low objective and adjust the lighting accordingly. Avoid using the stage clips. 
  3. Make observations about color, movement, body plan shape, etc… of the hydras by completing the chart and answering questions on the your “Result Data Sheet”.
  4. Remove the slide (clean, dry and return). Clean and put away all materials.




Result Data Sheet


Observation Table:  






(yes or no)





(brown or green)






(yes or no)




(hydrostatic or spicule skeleton)


Nerve Net Observable?


(yes or no)


Number of Tentacles












(small bumps on tentacles)


(yes or no)



Body Plan




(polyp or medusa)






(yes or no)


   Asexual  Observation?


(budding or no budding)





(yes or no)






(Cnidarian or Chordates)











1.      According to your resources (notes/text), describe at least three (3) characteristics that all Cnidarians, such as hydras, share:

2.      Of the two (2) body plans found in Cnidarians, what body plan(s) do hydras exhibit?

3.      What is the gastrovascular cavity slightly comparable to in a human?

4.      Besides hydras name two other types of cnidarians as discussed in class:

5.      What is the dart-like weapon that is used for offense and defense, called?

6.      With relation to movement what makes a polyp different from a medusa?

7.      What non-tissue layer is located between the endoderm and the gastroderm (ectoderm) of any cnidarian, including hydras:

8.      Green hydras have a symbiotic relation with what organism?

9.      Distribution of nutrients to cells in a hydra is done by simple what?

10.  Did you (and your partner) get similar data results when comparing your table to neighboring students? (Yes, No, Somewhat)