(a) Hormones - These are chemical substances secreted by endocrine/ductless glands, directly into the blood/ circulatory system, which on reaching a target organ, stimulate activity of that organ.

Hormonal	Nervous
Transmission is chemical	transmission is mechanical and chemical
slow transmission	Rapid transmission
Response is wide spread	Response localized
Target organs receive response	effector organs receive response
Long last response	short-lived response
pathway not specific through blood	pathway specific through nerves fibres

(c)(i) Effects of over-secretion: -

Over active/Hyperactive/Irritable/nervous.
Excessive growth/gigantism
Loss of weight
Enlarged thyroid gland/exothalmic goitre
Accelerated/increased heartbeat
Protrusion/bulging out of eyeball.

(ii) Effects of under-secretion: -

Cretinism/dwarfism in children.
Overweight/increase in subcutaneous fat/obesity.
Physical/mental sluggishness in adults.
Reduced heart rate.
Lowered/reduced metabolic rate/ myxceodema.
Goitre formation/hyperplastic goiter.

(d) Labels: Receptor organ, sensory neurone, intermediate/relay neurone; motor neurone, synapse, spinal cord, ventral root, dorsal root, effector organ, spinal nerve.