Topic 10: Kingdom Animalia - Introduction to animals

Characteristics of all animals
-eukaryotes
-multicellular
-heterotrophs
-have sexual reproduction (egg and sperm)

Development (embryonic development)
All animals begin life as a zygote (fertilized egg).  The zygote then divides into a mass of cells, called a morula (szedercsíra).  These cells continue to divide and they reorganize to form a hollow ball, called a blastula (hólyagcsíra).  The cells further reorganize, so that one side moves inwards to form gastrula (bélcsíra).  The gastrula has an opening at one end called the blastopore and the rest of the gastrula is either 2 layers (in the simplest animals) or 3 layers (in more complex animals) thick.  These layers are called germ layers, more specifically, the outer layer is ectoderm (külső csíralemez), the inner layer is the endoderm (belső csíralemez), and if there is a third layer, it is the middle layer, called the mesoderm.

Formation of the gastrula and germ layers

Once the gastrula has formed, the cells will begin to differentiate so that the cells will have different structures and functions and will form different tissues (szövet).  The tissues will organize to form organs and then organ systems.

Through evolutionary time, in the animal kingdom there has been a progressive increase in complexity.  The simplest animals have no symmetry, or are asymmetrical (like sponges).  As complexity increases, so does symmetry, first with the development of radially symmetrical organisms (such as jellyfish), and then bilaterally symmetrical organisms (like us!).

The majority of animals are invertebrates, meaning they do not have a bony, internal skeleton.  The invertebrates are about 95% of the animals on the planet!  The remaining 5% are the vertebrates, or those animals that have an internal skeleton.