In lower vertebrates (fish, amphibians) MSH from the pars intermedia is responsible for darkening of the skin, often in response to changes in background color. This color change is due to MSH stimulating the dispersion of melanin pigment in dermal (skin) melanophore cells.

Introduction:
MSH (melanocyte-stimulating hormone) is a hormone produced by the pars intermedia of the pituitary gland. It plays a crucial role in regulating skin coloration in lower vertebrates. In this question, we are asked to identify the effect of MSH on the skin coloration of lower vertebrates.

Explanation:
MSH is known to stimulate the production and dispersion of melanin, the pigment responsible for skin, hair, and eye color. In lower vertebrates such as amphibians and reptiles, MSH has been observed to cause darkening of the skin. This is primarily due to the following reasons:

1. Stimulation of Melanocytes: MSH acts on specialized skin cells called melanocytes, which are responsible for producing melanin. When MSH binds to specific receptors on melanocytes, it triggers a series of intracellular events that lead to increased melanin synthesis.

2. Increased Melanin Production: MSH stimulates the production of melanin by activating enzymes involved in its synthesis. These enzymes, such as tyrosinase, play a crucial role in converting the amino acid tyrosine into melanin.

3. Dispersion of Melanin Granules: MSH also promotes the dispersion of melanin granules within melanocytes. This dispersion allows melanin to be distributed more evenly throughout the skin, leading to a darker coloration.

4. Adaptive Response: The darkening of the skin in response to MSH is often an adaptive response to environmental factors such as temperature, light intensity, and camouflage. By increasing melanin production and dispersion, MSH helps to protect the skin from harmful ultraviolet (UV) radiation and regulate body temperature.

Therefore, the correct answer to the question is option 'A' - Darkening of the skin. MSH produced by the pars intermedia of the pituitary gland causes darkening of the skin in lower vertebrates by stimulating melanin production and dispersion in melanocytes. This adaptive response helps them to cope with environmental changes and maintain optimal skin function.

Pituitary gland -orchestra secretes Melanocytes Stimulating Hormone to induce pineal gland in its lobe part and we know that melanin gives dark colouration and hence A is answer