Animals

Concept : Animals are multicellular, heterotrophic eukaryotes that obtain nutrients by ingestion. they probably evolved from flagellated protist. Animal cells lack walls and are held together by unique proteins and junctions. animals are diploid ,except for eggs and sperm. their embryos go through a blastula and gastrula stage, and some pass through a larval stage before becoming adults. development is controlled by homeotic genes. most animals have three cell layers, muscle cells, and nerve cells.

 Figure 18.1

1. Male and Female adult animals make haploid gametes by meiosis
2. An egg and a sperm fuse, producing a zygote
3. The zygote divides by mitosis
4. Forming an early embryonic stage called a blastula, which is a hollow ball of cells
5. In the sea star and most other animals, one side of the blastula folds inwards, forming a stage called gastrula
6. The internal sac formed by gastrulation becomes the digestive tract, lined by a cell layer called the endoderm
• ectoderm - an outer layer that give rise outer covering if an animal
• mesoderm - embryonic layer which forms the muscles and most internal organs
7. Many animals develop directly into adults, but others such as sea stars develop into larval stages first. Larva is an immature individual that looks different from the adult animal.
8. The larva undergoes a major change of body form, called metamorphosis, in becoming an adult capable of reproducing sexually.