Frogs are beneficial for mankind because they eat insects and thus, protect our crops. They serve as an important link of food chains and hence food web in the ecosystem, thus maintaining the ecological balance. Some countries use the muscular legs of frog as a food source.

Tissue is the missing in the chain. Structural organisation can be shown as cell-tissue-organ-organ system. Number of cells together form tissue, number of tissue together form organ which on when unit with several organ form organ-system. In organisms like Hydra, the body is comprised of thousands of cells in which each cell works independently whereas in a complex body system as that in humans billions of cells perform various functions together via connecting each other through connecting tissue.

Stratified epithelium consists of epithelial cells in which the deepest layer is made up of columnar or cuboidal cells. It is a type of compound epithelium and outer few layers, a water proof protein called keratin is present. These layers of dead cells is called horny layer which is shed at intervals due to frictions hence, has a limited role in secretion and absorption. The main function of stratified epithelium is to provide protection to the body against mechanical and chemical stresses.

Gap junctions facilitate intercellular communication by allowing small signaling molecules to pass from cell to cell. These are fine hydrophilic channels between two adjacent animal cells that are formed with the help of two protein cylinders called connexus. Each connexus consists of six proteins subunits that surround a hydrophilic channel. pH and $\mathrm{Ca}^{2+}$ ion concentration controls, opening or closing of channels.