Final answer:
In biology, excreting unused eggs refers to various reproductive mechanisms where unfertilized eggs are either absorbed by the female's body or passed into the environment. This occurs across a range of species, from monotremes with cloaca to humans and parasites. Unfertilized eggs might also develop into offspring in species that exhibit parthenogenesis.
Step-by-step explanation:
When discussing the process by which animals excrete unused eggs, this typically refers to the biological mechanisms involved in the reproductive systems of various species. In female monotremes, for example, they do not have a uterus or vagina but instead possess a cloaca which serves the purpose of excreting wastes and laying eggs. Similarly, there are mechanisms such as in ovoviparity, where fertilized eggs are kept within the female and given nourishment from the egg's yolk until they are ready to hatch. If these eggs are not fertilized, they have different fates depending on the species. In some cases, unfertilized eggs might be reabsorbed by the female's body, while in others, they may be passed into the environment. Notably, certain species such as some insects and reptiles can lay parthenogenic eggs, which can develop without fertilization.
In the context of parasites like Ascaris, the term 'unused eggs' could refer to embryonated eggs not reaching the host's intestine to develop into larvae. Otherwise, when referencing the human female reproductive system, if an ovulated egg is not fertilized, it will disintegrate after passing into the uterus. This process of unfertilized eggs passing through the reproductive system without leading to offspring can encompass various phenomena within different species and contexts.