Larger animals tend to reproduce sexually and smaller organisms reproduce asexually. Larger animals have developed more complex organ systems and with these organ systems they can adapt to their environment more easily than smaller organisms. The complex brain and sense organs of larger organisms allows them to adapt to their environment.
Organisms that reproduce asexually cannot develop much variety, because they are "copying" the original organism almost exactly. Sexual reproduction allows for great diversity, because the zygote is different from the mother's egg and father's sperm; it is a combination of both. Sexual reproduction produces a greater chance of variation within a species than asexual reproduction would. This variation improves the chances that a species will adapt to his environment and survive.
Heliozoa, Amoeba, and Euglena all reproduce by binary fission, which is the mother cell dividing into two daughter cells. The Heliozoa and Amoeba belong to the Protista Kingdom. The Euglena is an odd one celled plant that sometimes have characteristics of a protist. Heliozoa live in fresh water and have pencil like axopods that aid in eating. Amoebas live in fresh water, and move in a unique manner. An amoeba will move its entire body in a "blobby" motion. Euglenas live in fresh water and have a long tail that helps move the organisms through the water.
Planaria, round worms, and leeches are mostly hermaphroditic. The male and female reproductive systems are distinct, but may join terminally in a common chamber on the same organism. Self fertilization is rare. Some of the representatives of these groups are parthenogenetic: females asexually produce females. Planaria also are capable of extensive regeneration.
Gorillas, elephants, rats, zebras, and dolphins are all mammals that reproduce sexually. There is a male and female in each of these species. Kangaroos are mammals but they are marsupials. There are female and male kangaroos, but after sexual reproduction the fetus leaves the mother and goes into her pouch until it is large enough to leave.
Fish and frogs have sexual reproduction, but it is externally. The female lays eggs and the male externally fertilizes the eggs but squirting sperm in the water. Frogs develop differently, in that they have a tadpole stage and them metamorphose into a frog.
A turtle lays eggs, but like a bird and probably a dinosaur, fertilization occurs internally. Dinosaurs are extinct, but because we have found dinosaur eggs, we believe that they reproduced much like chickens. The male internally fertilized the female, and then the female laid her eggs.