Each time I find a New Zealand flatworm, or more usually two, under the plastic basin that sits on mossy tarmac at the corner of the house, I want to reinstate the Theory of Spontaneous Generation. Before resorting to this a few other theories need to be ruled out. One is that the moss conceals the opening of a tunnel which leads through the centre of the Earth to New Zealand. Another is that a Terricolan Internet exists through which A. triangulatus can book a few days in a quiet spot for undisturbed copulation. The most difficult theory to rule out, but which still stretches credulity, is the one woven from the available scientific facts: the flatworm manages to find the basin using its sense organs, nervous system and muscles.
A triangulatus can move at 17m per hour using the layers of circular and longitudinal muscle which lie beneath its thin ciliated epidermis. The slime it produces protects it from abrasion so it is undaunted by rough tarmac. I suspect its forays take place mostly at night. Because of its disgusting slime it appears to have no Irish predators apart from ground beetles, rove beetles and humans, but it depends on maintaining a moist epidermis to absorb oxygen and to eliminate the waste products of its metabolism, viz carbon dioxide and ammonia.
With their small pigment-cup eyespots we can’t expect flatworms to have a pictorial view of the world, although they can, presumably, distinguish light from dark. They have sensory pits on either side of their head, so it is likely they are attracted to earthworms by their irresistible aroma. What attracts them to moss under basins with leeks growing in them is anybody’s guess.
Once A. triangulatus finds itself in contact with its prey, out comes the reversible, muscular, pumping, enzyme-producing pharynx which until now has lain sheathed on the ventral side of the animal. The unfortunate earthworm is wrapped in a sticky secretion that is derived from slime and, perhaps, from the rhabdites, dark-staining, rod-shaped bodies found in the epidermal cells. This secretion is slightly acid and its enzymes may soften the earthworm’s cuticle and begin digestion before the prey is sucked into the flatworm’s three-branched, cul-de-sac of a digestive system. The mouth of A. triangulatus also acts as its anus.
If food is unavailable, flatworms begin to consume their own organs, starting with unlaid eggs, proceeding through the yolk glands to the rest of the hermaphrodite reproductive system. Next to provide sustenance is the parenchyma, cells that lie between the epidermis and the gut. After that the gut is broken down, then the muscles. The epidermis and the nervous system alone are spared. In this way A. triangulatus can survive for a year without food. When this becomes available again the lost organs regenerate and the flatworm returns to its normal size.
A lesser degree of specialisation can have its advantages.
