Planaria
Planaria can reproduce either sexually or asexually. Those that produce sexually possess both testicles and ovaries. Thus, one of their gametes will combine with the gamete of another planaria. Each planaria transports its excretion to the other planaria, giving and receiving sperm. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. Sexual reproduction is desirable because it enhances the survival of the species by increasing the level of genetic diversity. In asexual reproduction, a planaria detaches its tail end and each half regrows the lost parts by regeneration, allowing neoblasts (adult stem cells) to divide and differentiate, thus resulting in two worms. Some species of planaria are exclusively sexual or asexual, whereas some can reproduce both sexually and asexually.