The life cycles of many familiar animals, like mammals and birds, appear less intricate than the life cycles of plants and fungi. As you will see, the life cycles of most animals are more complex than those of birds and mammals, but animals in general still lack some complications seen in other groups. The main simplifying factor is the lack of any alternation of diploid and haploid generations, at least of the type seen in plants.
All animals have gametic meiosis , meaning that meiosis leads directly to the production of gametes.
There is no independent haploid stage in the life cycle. Successful gametes unite to form a diploid zygote, which develops into an embryo and then into a functional organism. If this organism is lucky enough to survive, it may grow to maturity and produce gametes of its own. The processes of fertilization , development , growth , maturation , reproduction , and senescence occur in nearly all multicellular organisms, so some general principles apply to the life cycles of fungi, algae, plants and animals alike. However, each of these groups has its peculiarities, and even within each group special circumstances contribute to differences in life cycles.