Lack of a rigid cell wall: The lack of a rigid cell wall allowed animals to develop a greater diversity of cell types, tissues, and organs. Specialized cells form different tissues and organs that carry out specific functions. Protozoa can move, but it is only with the help of cilia, flagella and pseudopodia, not muscles as in the case of animals.
Collagen: The animal kingdom is unique among eukaryotic organisms because most animal tissues are bound together in an extracellular matrix by a triple helix of protein known as collagen. Plant and fungal cells are bound together in tissues with the help of pectin.
Hardened structures: Bones, shells, spicules, and other hardened structures are formed when the extracellular matrix containing collagen between animal cells becomes calcified.
Main organelles and composition of an animal cell: The main organelles and composition of an animal cell include Plasma membrane, Cytoplasm, Nucleus, Endoplasmic reticulum, Ribosomes, Golgi bodies, Lysosomes, Mitochondria, Outer Membrane, Intermembrane Space, Inner Membrane, Matrix, Vacuoles, Cytoskeleton