Flatworms
James H. Thorp, D. Christopher Rogers, in Field Guide to Freshwater Invertebrates of North America, 2011
B Reproduction and Life History
Some flatworms, especially those living in temporary aquatic habitats, reproduce only once per year (univoltine), while other species are multivoltine, with the number of generations depending on environmental favorability. Most flatworms reproduce sexually, and almost all species are hermaphroditic. Although gamete exchange is most common, self-fertilization is possible in some species. Eggs are laid singly on a hard substrate, are grouped into clusters (sometimes stalked), or are placed in cocoons. Miniature flatworms emerge from eggs which have been attached to the substrate. These usually develop directly (without intermediate larval stages) into adults and differ only in size and presence of reproductive organs. A few species are ovoviviparous (young brooded inside parent) or have a distinct larval stage. Asexual reproduction occurs seasonally in many taxa and is the only form of reproduction in a few species. This form of reproduction involves differentiation of the adult body into distinct units (zooids) followed by separation into small flatworms via transverse division of the adult body. This process is aided by the remarkable regenerative abilities of flatworms.
Under demanding environmental conditions, flatworms produce resistant eggs able to survive desiccation of the surrounding environment or the animal may enter diapause (a very low metabolic state) and encyst as a whole animal or fragment of the adult.
Sexually reproducing flatworms can live from a few weeks to a few months (not counting periods of encystment), while the life span of asexual species is theoretically indefinite.