Animal

**Etymology and Terminology:**
– The word ‘animal’ comes from the Latin noun ‘animal’ meaning having breath or soul.
– ‘Metazoa’ is derived from Ancient Greek words meaning after animals.
– The term ‘animal’ refers to all members of the kingdom Animalia.
– In colloquial use, ‘animal’ often refers to nonhuman animals.

**Characteristics and Structure:**
– Animals are eukaryotic, multicellular, and heterotrophic.
– They respire aerobically and are motile at some stage of their life cycle.
– Animal bodies are composed of cells with an extracellular matrix.
– They have tissues, muscles, nerve tissues, and an internal digestive chamber.

**Reproduction and Development:**
– Sexual reproduction is common among animals.
– Animals produce haploid gametes by meiosis.
– Zygotes develop into blastulas, which can further develop into new organisms.
– Germ layers differentiate to form tissues and organs in most animals.

**Ecology and Diversity:**
– Animals are categorized based on trophic levels.
– They consume organic material and play vital roles in food webs.
– Predation leads to evolutionary arms races between predator and prey.
– Some animals exhibit specific feeding behaviors, contributing to ecosystem balance.

**Size, Numbers, and Phylogeny:**
– The blue whale is the largest animal, while microscopic animals like Myxozoa are tiny.
– The estimated number of animal species is around 7.77 million.
– Animals are monophyletic, with origins tracing back to unicellular ancestors.
– They are divided into non-bilaterians and bilaterians, with distinct characteristics and evolutionary relationships.