H istory of classification

Reproduction and development

See also: Sexual reproduction § Animals and Asexual reproduction § Examples in animals

The use of love darts by the land snailMonachoides vicinus is a form of sexual selection

Nearly all animals undergo some form of sexual reproduction.^[19] They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova.^[20] These fuse to formzygotes, which develop into new individuals.^[21]

Many animals are also capable of asexual reproduction.^[22] This may take place through parthenogenesis, where fertile eggs are produced without mating, budding, or fragmentation.^[23]

A zygote initially develops into a hollow sphere, called a blastula,^[24] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.^[25] In most other groups, the blastula undergoes more complicated rearrangement.^[26] It first invaginates to form a gastrulawith a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm.^[27]In most cases, a mesoderm also develops between them.^[28] These germ layers then differentiate to form tissues and organs.^[29]