1.  A receptor receives information from the environment.
 They can be classified according to the type pf stimulus
they respond to.
 Receptors are traducers: they convert the
Energy of the stimulus into the kind of
energy the nervous system can deal with
(electrical impulses).
Receptor type Responds to stimulus Example in humans
Photoreceptor Light Rod cells in retina of eye
Chemoreceptor Chemicals Taste buds
Thermoreceptor Changes in temperature Thermoreceptors in skin
Mechanoreceptor Mechanical changes Hair cells in ear.

3.  Unicellular organisms and plants.
Photoreceptor pigment inside the cell.

4.  Worms

5.  Planarian
Light sensitive
eyespots (ocelli)
Only sense variations in
light´s intensity.
They can detect where the
light is coming from.

6.  Mollusks
Discriminates simple shapes.

8.  Receptors may need help in receiving the
stimulus. They are often grouped together with
other tissues to form a sense organ. The other
tissues allow the receptor cells to work
efficiently.

9. Simple eye
Processes some
images and
detects shapes.

10.  Insects
Compound eyes: consists of numerous separate light-
sensitive units (ommatidia). All these images
generate an overall picture.
Very useful in detecting movement.

11.  Vertebrates

13.  Laterally located eyes: the animal perceives a
wide picture of its surroundings.

14.  Frontally located eyes. 3D imaging allows
better asessment of target´s position and field´s
depth.

15.  Find the function for the underlined structures: