This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.
An animal is any living organism specifically characterized by the act of voluntary movement, presence of cells containing non cellulose cell walls together with specialized sense organs that creates the rapid response to stimuli, and complex organic substances ingestion, for instance the plants and other animals. Animals compose of a major group of the multicellular organisms belonging to the kingdom Animalia also known as Metazoa. Their body organization turns to be eventuall fixed as they develop. However, some animals undergo the metamorphosis process in their growth. Most of the Animals are Motile showing the ability to move independently and spontaneously. They are also heterotrophs to mean that they ingest other orgarnisms for the purpose of sustenance. Animals are different from other living organisms. Being eukarytic and multicellular differentiates them from bacteria and protists. Unlike plants and algae, animals are heterotrophic hence they diget food in an internal chamber. Their cells lack rigid cell walls a feature that distinguishes them from algae, plants and fungi. All animals can move from one place to another showing that they are mobile. They are also motile except at some stages in their life. Most of the animals have embryos passing through blastula stage, a characteristic found in no other living thing. In unicellular protozoa, the animals are known to be accelular, microscopic and eukaryotic. They live in water and exhibit holozic or parasitic nutrition. Their locomotion is through pseudopodia, flagella and cilia, and it may be lacking sometimes. The reproduce through binary fission, meiosis and also syngamy, hence sexual reproduction same with other animals. In phylum porifera, simply known as the sponges, they lack clear symmetry and specifically itâ€™s the only group of animals lacking at least two differentiated embryonic tissue layers. This aspect has led them to be classified as parazoa next to animals. Some of their cells are specialized for nutritional and reproductive purposes, an aspect that makes them hold on the edge of animal kingdom. Unlike other animals, sponges lack the digestive gut. The outer surface contains spores in which water flows into the central space and flows out via the large opening at the top of the sponge. This water flow brings oxygen and food and carries away carbon dioxide and other wastes. They all secrete a skeleton that helps in maintain their shapes. For the phylum cynidaria, the organisms have radial symmetry, for example the jelly fish and hydras. They posses a true digestive gut same with all other animals. The difference in this is that the opening serves as the mouth and also as the anus. Their body layers are made of two layers of cells which are the ectoderm and the endoderm. All animals are classified according to the characteristics listed above. Non animals do not posses these given characteristics.
Phagocytosis is a process through which phagocytes ingest and engulf food substance such as including other cells and particles. The phagocyte cell appears to a free-living organism containing one cell at sometimes, for instance the amoeba, or at other times one among the body cells, for instance the white blood cells also referred to as leukocyte. Phagocytosis is used as a means of feeding in some of the animal life such as sponges and amoebas. This process begins with hand like projections extending from the plasma membrane. These membranes contain phospholipids made of two sections, the polar phosphate with acts as the head and the non polar fatty acids which is the tail. The molecules are arranged into a double layer known as bilayer, making the head to face the internal and external of the cell, and the fatty acid forming the internal of the cell membrane. The phospholipids are made in a way that they move freely mostly switching neighborhood places giving a chance for the membrance to stretch hence changing shape. The plasma cell projections surrounds the food particle and hence fuse together to engulf the particle completely in a large cell vesicle referred to as phagosome. The phagosome fuses with a membraneous organelle inside the cell known as lysosome to form one membraneous organelle in a process in which they mix their contents. Lysosomes are sack like structures containing hydrolytic enzymes as digestive enzymes that are used in the breakdown of macromolecules into small molecules and serve as a defensive structure. Also, they act as the cell stomach. Once the phagosome fuse with the lysosome, the ingested material is broken down minute molecules by these enzymes and hence taken to the cytosol for cell use. Other cells involved in the process of phagocytosis include the Golgi apparatus, cytoskeleton, ribosomes, cilia and flagella, food vacuole and also the contractile vacuole. The Golgi apparatus structure is made of cisternae, flattened membrane stacks in a series of five to eight. These cisternae assist the transport of proteins in different cell parts. The Golgi apparatus are responsible for macromolecules handling that are used in proper functioning of the cell. It serves to processes and packs the macronutrients for use and secretion in the cell. They also modify proteins coming from rough endoplasmic reticulum and transports lipids to the cell vital parts and also create the lysosomes. Cytoskeleton is a framework of hollow tubes made of proteins. It supports the cell structure and contains the cilia. Cilia are short threadlike structures to enhance their cell movement. They serve to move substances along the cell surface. Also, the cytoskeleton contains the flagella. Flagella are whip like structures for cell movement. Ribosomes are structures involved in protein synthesis. The vacuoles are membrane enclosed sacs used as a site for storage of materials such as water and proteins.
Development stage begins with fertilization. Fertilization is the process of fusion of the egg nucleus and the sperm to form a zygote. Here, it starts with the fusion of gametes, that is, the egg and the sperm. The sperm swims to the egg piercing its cell membrane and creating its way into the cell. In the fertilization, egg and sperm genetic material are combined. Is gamete presented is known to be haploid (containing half of the chromosomes). After the fertilization, these gametes combine together forming a zygote. Immediately the sperm enters the egg, the cell membrane adheres to certain changes preventing additional entrance of sperms. This is the process of cleavage. The chromosomes combine together and first cell division occurs. The ready fertilized egg divides into blastomers, which are two daughter cells. More divisions occur from this step with each cell dividing into two. This doubling of cells gives a quick rise to a multicellular embryo. Within a short time, the embryo forms more than one hundred cells known as the morula. These are arranged inform of solid ball of blastomers. The morula cells rearrange into one layer enclosing a central cavity filled with fluid. The embryo is referred to as the blastula at this stage. This is followed by the folding of blastula cells at blastopore and the creation of gastrula by invagination. The gastrula composes of cup of cells made of two layers. The outer layer is referred to as the ectoderm while the inner layer the endoderm. The inner layer covers the primitive gut. Mesoderm develops as a third layer in between the endoderm and the ectoderm. These three germ layers is where the cells, organs and tissues develop. Ectoderm cells differentiate into epidermis, nails, hair, sweat glands, claws, brain, tooth enamel and the spinal cord. The mesoderm divides into blood, muscles, blood vessels, heart, kidneys and reproductive organs. For the endoderm, it differentiates into respiratory and digestive system lining cells, gall bladder, the pancreas and the liver. Blocks to polyspermy are among the early stages of development. This serves to prevent the two sperm nuclei incorporation into zygoteâ€™s genome, which may cause an abnormal development. To prevent polyspermy, the fertilized eggs use blocks at the zona and plasma membrane. However, contributions of these blocks are known to vary in different species. However, in cleavage stage, the pattern is influenced by the amount of York and mitotic spindle formation. York is the nutrient material which is stored in the egg. The York impedes the cleavage furrow formation. If an embryo has little or lacks York, its daughter cells appear to be of similar size. This is mostly in sea urchin. When the quantity of York is high, there is asymmetry of cell size. Despite this, the increased amount of York results into incomplete cleavage, as the furrows of the cleavage do not penetrate the York. In this case, the embryo creates disc of cells on the top of the York. This disc of cells is called blastodisc and the incomplete cleavage is called discoidal cleavage.
Indirect development is an adult development through a sexually immature stage. The fertilized egg is divided to produce a hollow and ciliated ball of cells, referred to as blastula. The cleavage appears to be total, radical and also indeterminate. However, the blastula invaginates on its one end forming primitive gut. The cells hence continue with the process of dividing to create a double layer of embryo known as gastrula. The larval stage is interposed in between the adult and the embryo and undergoes metamorphosis. The larva is different from the parent and hence can use different feeding and habitat. This helps in preventing competition for same resources hence increased growth in population. Direct development on the other hand involves no distinctive larval stage. When the young animals are born or hatched, they little resemble their parents. They do not undergo metamorphosis stage and the sexually immature animals can be referred to as nymphs. These have the advantage of new habitat colonization and geographical range expansion. There is also the avoidance of catastrophe which is associated with failure of local habitat. Triploblastic is a condition in which there is presence of three germical layers in the ovum. These layers are the ectoderm, mesoderm and the endoderm. It also refers to an ovum with three layer blastoderm. All animals are known to come out of tripoblastic ovum. Platyhelminthes are the simplest animals with triploblastic conditions. These are flatworms with no body cavity except the gut. They lack the anus with same pharyngeal opening acting as both the mouth and the anus. Due to the lack of the body cavity, the gut is highly branched in larger flatworms for effective transport of food to all body parts. Their flat shape is due to lack of cavity hence they respire through diffusion. They are divided into Turbellaria, Trematoda and Digenea.