Question

Explain the process of regeneration in Planaria. How is this process different from reproduction?

Answer 1

A planarian is one of many flatworms of the traditional class Turbellaria. It usually describes free-living flatworms of the order Tricladida although this common name is also used for a wide number of free-living Platyhelminthes. Planaria exhibit an extraordinary ability to regenerate lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. Some planarian species have two eye-spots also known as ocelli that can detect the intensity of light, while others have several eye-spots.

Step by step answer: Planaria can be cut into pieces, and each piece can regenerate into a complete organism. Cells at the location of the wound site proliferate to form a blastema that will differentiate into new tissues and regenerate the missing parts of the piece of the cut planaria. It's this feature that gave them the famous designation of being "immortal under the edge of a knife."
New tissues can grow due to pluripotent stem cells that can create all the various cell types. These adult stem cells are called neoblasts and comprise 20% or more of the cells in the adult animal. They are the only proliferating cells in the worm, and they differentiate into progeny that replace older cells. Also, existing tissue is remodeled to restore symmetry and proportion of the new planaria that arise from a piece of a cut-up organism.
If the head of a planarian is cut in half down its center, and each side retained on the organism, the planarian can regenerate two heads and continue to live. In contrast, two-headed planaria regenerates can be induced by exposing amputated fragments to electrical fields. Such exposure with opposite polarity can induce a planarian with 2 tails. Two-headed planaria regenerates can be induced by treating amputated fragments with pharmacological agents that alter levels of calcium, cyclic AMP, and protein kinase C activity in cells. Reproduction in Planaria is different from other processes of reproduction as they involve the fusion of gametes which result in the formation of a zygote which later develops into an individual.

Note: The planarian has very simple organ systems. The digestive system consists of a mouth, pharynx, and a gastrovascular cavity. The mouth is located in the center of the underside of the body. Digestive enzymes are secreted from the mouth to begin external digestion. Planaria receive oxygen and release carbon dioxide by diffusion. The excretory system is made of many tubes with many flame cells and excretory pores on them.