A planarian can regrow any missing body part. Truth be told, in the event that you take a planarian and excise it into a few little pieces, every last one of those body sections can recover into an entirely different worm.
Calcium levels in the body are maintained by hormones. If the blood calcium level rises above a set point, calcitonin is secreted by the thyroid gland and sets into motion responses to lower the blood calcium back to the set point. If the blood calcium level falls below a set point, the parathyroid gland secretes parathyroid hormone that sets into motion responses to raise the blood calcium. These homeostatic responses are examples of:
a. positive feedback
b. effectors
c. sensors
d. negative feedback
Answer:
d. negative feedback
Explanation:
Negative feedback responses are the ones that counteract the change. Any alterations in the internal body conditions such as blood calcium levels are sensed by receptors. The information is sent to the brain which in turn commands the effectors such as endocrine glands to secrete the required hormones to restore the normal conditions. Here, changes in the calcium levels are counteracted by the secretion of calcitonin and parathyroid hormones and the calcium is maintained within a narrow range. This represents negative feedback.
Absolute time measurements can be used to calibrate the relative time scale, producing an integrated geologic or <em>"geochronologic"</em> time scale. The overall duration and relative length of these large geologic intervals is unlikely to change much, but the precise numbers may "wiggle" a bit as a result of new data.
