What are the two main communications systems in an animal's body?

Name two parts of the body which contain receptors of chemical stimuli.

Which part of the eye contains cells which are sensitive to light?

Which one term in each of the following includes the other three?

(a) thyroid, ductless gland, thymus, pituitary, ovary

(b) adrenalin, insulin, hormone, thyroxine, estrogen

Which parts of the body form the central nervous system?