What is an Animal?

Characteristics of Animals Animals are eukaryotic, multicellular organisms Cells do not have cell walls Can move in some way All animals are heterotrophic: must consume food (autotrophs like plants or other heterotrophs like hamburger from a cow) to obtain energy. Digest food Have specialized cells

Development Animals share similar developmental stages. –Fertilization: Sperm (1N half the genetic material of the male adult) joins with the egg (1N half the genetic material of the female adult) The fertilized egg is called a zygote. –Cell division: The zygote divides through the process of cleavage. Once the cell division starts the name changes from zygote to embryo. –The cells continue to divide until they form a hollow ball of cells called the blastula. The hollow space is called the blastocoel. –The cells on one side of the blastula buckle inward to form the gastrula.

Development continued The layer of cells on the outer surface of the gastrula make the ectoderm. This continues to grow and eventually becomes the skin and nervous system. The layer that folds in on the gastrula is called the endoderm. The endoderm continues to grow and the cells eventually differentiate into the digestive system and digestive organs. Some animals develop an additional layer between the ecto and endoderm. The cells come from the endoderm and the new middle layer is called the mesoderm. The mesoderm becomes the muscular, respiratory, circulatory, and excretory systems.

Development continued From the gastrula on the cells continue to grow and differentiate into tissues, organs, and systems. If the opening (blastopore) formed by the invagination (buckling) of the gastrula becomes the mouth the animal is called a Protostome. EX. Earthworms, insects If the mouth of the animal develops from another area (not the opening, or blastopore) on the gastrula it is called a Deuterostome.

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