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Of the worms, the Platyhelminthes ("flat worms") are considered the prototypes,   having developed such innovations as bilateral symmetry, a head, tail, and three germinal tissue layers (stinging celled animals, comb-jellies... only have two). These simple soft-bodied animals use their skin to breath through and only have one body opening, the mouth serving also as an anus.

Platyhelminths are true hermaphrodites... being both male and female. Cross-fertilization is the rule with a "pair" rudely stabbing one another with a hard penile stylet to enact sperm transfer. As you know due to exposure with Planaria, they can also generate new individuals by splitting asexually.