Animal Diversity - Intro PDF

The animal classification system characterizes animals based on their anatomy, morphology, evolutionary history, features of embryological development, and genetic makeup.  Most members of the animal kingdom share certain features. Animals are eukaryotic, multicellular, heterotrophic organisms that...

The animal classification system characterizes animals based on their anatomy, morphology, evolutionary history, features of embryological development, and genetic makeup.  Most members of the animal kingdom share certain features. Animals are eukaryotic, multicellular, heterotrophic organisms that ingest their food and usually develop into motile creatures with a fixed body plan. A major characteristic unique to the animal kingdom is the presence of differentiated tissues, such as nerve, muscle, and connective tissues, which are specialized to perform specific functions. Most animals undergo sexual reproduction, leading to a series of developmental embryonic stages that are relatively similar across the animal kingdom. **[Body symmetry ]** All true animals except those with radial symmetry are bilaterally symmetrical. **Asymmetrical** is no symmetry at all. **Radial symmetry** is the arrangement of body parts around a central axis. It results in animals having top and bottom surfaces but no left and right sides, or front or back.  **Bilateral symmetry** involves the division of the animal through a sagittal plane, resulting in two mirror images and right and left halves. **[Germ layers]** Most animal species undergo a separation of tissues into germ layers during embryonic development. Recall that these germ layers are formed during gastrulation, and that they are predetermined to develop into the animal's specialized tissues and organs. The animals that display radial symmetry develop two germ layers and those with bilateral symmetry develop three tissue layers. **Diploblasts** develop two germ layers, an inner layer (endoderm) and an outer layer (ectoderm). Diploblasts have a non-living layer between the endoderm and ectoderm. **Triploblasts** develop three tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). **[Internal body cavity]** Further subdivision of triploblasts results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a coelom, and those that do not. This epithelial cell-lined coelomic cavity represents a space, usually filled with fluid, which lies between the visceral organs and the body wall. **Acoelomates** -- triploblasts that do not develop a coelom. Their mesoderm region is completely filled with tissue, although they do still have a gut cavity **Coelomates** -- animals with a true coelom. A true coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion. **Pseudocoelomates** -- animals with a coelom derived partly from mesoderm and partly from endoderm, which is found between the two layers. Although still functional, these are considered false coeloms. **[Digestive cavity]** Bilaterally symmetrical, tribloblastic, coelomates can be further divided into two groups based on differences in their early embryonic development. These two groups are separated based on which opening of the digestive cavity develops first: mouth or anus.  **Protostomes** -- mouth develops first **Deuterostomes** -- anus develops first