What is a stimuli in terms of feed back loops

In the feed back loop of a homeostatic control system, stimuli means a change in the environment which causes a change to the factor being regulated.

Explanation:

Homeostasis is the tendency of the various systems in the human body to stay in optimal ranges for health by self regulation through feedback controls. It is very important because it maintains equilibrium and provides stability to the human body. In a feedback loop, the output of a system enhances the system or restrain the system.

In a homeostatic control system, stimulus means a change in the environment. It causes a change to the factor being regulated. Receptor detects this stimuli and responds to it.  This response acts as an input and travels to the control center. The control center decides the course of action and send the output information to the effector (muscles, glands). The effector responds to this output and maintains homeostasis by balancing the original stimulus.

In blood sugar regulation, the rising level of blood sugar (stimuli) is sensed by the receptors in the body. The response of the receptors is then  passed to the pancreas. Pancreas secretes insulin into the blood and lowers the blood sugar levels. The pancreas stops insulin secretion once the blood sugar levels reach homeostasis.

A negative feedback loop (inhibitory loop) is a type of self-regulating system in which increased output from the system inhibits the system's  future production. Example of negative feedback to achieve homeostasis are blood pressure , body temperature,  blood sugar.

In a positive feedback system, the original stimulus is amplified by the output. Examples of a positive feedback system are child birth, lactation, blood clotting etc. During child birth, the hormone oxytocin released increases and speeds up the contractions.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.

substitution is the answer in this case