The weirdly shaped "hammerhead worm" is a type of predatory land planarian (Bipalium kewense). It is thought to be native to Asia, but the species was first described in Kew Gardens, and it now has a cosmopolitan distribution, most likely due to the potted plant trade. In the U.S., it is often associated with greenhouse and nursery plants. Here in Texas, it lives under rocks and near homes and gardens where the dirt is sufficiently moist. During dry periods, it curls up in a little retreat of slime, but ventures forth whenever conditions are wet. This large flatworm (so described as it can easily be several inches in length) preys mostly upon earthworms and is considered a pest where worms are raised commercially.
While the planarian is hermaphroditic, it can also reproduce by segmentation: when it is severed, each piece will grow back the missing parts. This isn't as difficult as it seems; the creatures lack both respiratory and circulatory systems. They have no known predators, probably due to their sticky slime and the neurotoxins they produce, the latter of which might also help them subdue their own prey.