Animals are composed of eukaryotic cells. Animal cells lack a cell wall and have the capacity to adhere to one another to develop into multicellular tissue or organ. As they lack a cell wall, the cells are flexible. Eukaryotic cells possess well-developed membrane-bound cell organelles like endoplasmic reticulum, nuclei, and mitochondria. The cells communicate with each other through various intracellular and extracellular pathways. Cells adhere with one another through various junctions such as tight junctions, gap junctions, adherens junction, desmosomes, non-classical junction, and focal adhesion molecules. Cells adhere through these junctions and develop into tissues. Each tissue has specific cells that have specific functions. The tissues develop into organs that perform a specific function. During the process of fertilization the zygote, consist of stem cells that develop and specialize into four different types of tissues such as connective, nervous, epithelial, and muscle tissues. Some advanced organisms have highly specialized cells such as nerve cells, liver cells, and pancreatic cells. The nerve cells adjoin to develop into the nervous system. The nervous system is a connector to various receptors throughout that help the animal in sensing the environment. Animals can sense smell, taste, vision, touch, and hearing due to the coordination of the receptors and nervous system.
Fertilization and Development of Animals
Animals multiply through sexual reproduction. Sexual reproduction allows the recombination of the genetic material and develops offspring that contain the genetic information of both the parents. This special ability leads to the development of new characters and adaptation in animals according to their needs. Sexual reproduction happens through the combination of male and female gamete which outcomes in the generation of a zygote. The zygote undergoes various stages such as morula, blastula, and gastrula. The morula stages happen after a few divisions after the generation of the zygote. These cells undergo several divisions and form into a hollow sphere of cells that contains an internal fluid called a blastula. The cells of blastula again undergo various cell divisions and differentiate themselves into three layers such as ectoderm, endoderm, and mesoderm that differentiate into various organs during the stage of development. This stage is known as gastrulation. The cells further differentiate and develop into organisms with well-developed organs system. Nutrition and movement: Animals are heterotrophs, unlike plants they rely on other sources for food. They are not able to synthesize their food. They depend on plants and other animals to provide nutrition. Proper nutrition helps the animals to perform bodily functions without any restriction. The digestive system processes the consumed food into absorbable nutrients in the stomach. The nutrients are absorbed by the small intestine which reaches every cell of the body through the bloodstream. The waste is removed out of the body through the excretory system. Animals are mobile and can move from one place to another. All animals are mobile at least at one stage of their life. Their ability to move enables them to find food, protect themselves, and move to a new habitat.
