Found under a log: two wonderful worms and their weird details

2013-10-03 00:00

WHILE overturning rotten logs at Queen Elizabeth Park (QEP), hunting for interesting creatures that live in such places, I came across two really interesting animals that keen wildlife enthusiasts should know about. One was a flatworm and the other a velvet worm. Let me tell you a little about these weird worms.

Flatworms belong to a group called Platyhelminthes, while velvet worms are Onychophora. These groups have very little in common, except that the two I found live under similar conditions and are both nocturnal predators that hunt, kill and eat small invertebrates.

My flatworm belongs to a subgroup called the Turbellaria, a group described as being “free living”, mainly because most of the other flatworms are internal parasites and include things such as tapeworms. Those that are free living are mostly aquatic. However, the highly elongate species that I found is one of the terrestrial worms that live in damp places such as leaf litter. While you commonly find them in forests, they may also be found in damp compost heaps. In my limited experience, these elongate worms, with characteristic spade-shaped heads, are usually about 15 cm to 25 cm in length. They are difficult to measure because they are capable of stretching rather like a rubber band. I believe they can grow to 50 cm and even 60 cm, but I have never seen one that size. These worms are reputedly the simplest bilaterally symmetrical animals on Earth. So basic is their structure that they lack many features found in other animals. They have no body cavity, but have a mouth and a gut. Most species, except the long ones, lack an anus, which means that undigested material has to be regurgitated. Food is absorbed by simply diffusing into the body cells. They have no respiratory organs so oxygen also simply diffuses into the body — this is the main reason they are flat, they need a big surface area. They have a very simple nervous system and tiny photosensitive spots that act as eyes, only capable of detecting light and dark. Little is known of their feeding habits, but they produce lots of sticky mucus with which to immobilise prey. Their mouths are eversible (i.e. can turn inside out) and so they spill digestive juices onto their prey, let the juices digest the prey’s body and then suck up their predigested meal. They have simple muscles but propel themselves along is an extraordinary manner. The undersides of their bodies have myriad tiny mobile hairs called cilia, which, when waved backwards and forwards in their slimy mucus, allows them to literally glide along on slime. Perhaps the most fascinating thing about flatworms is that they are masters of regenerating themselves whenever necessary. For example, if you cut a worm into a few pieces, each piece will regenerate a head at one end, a tail at the other, and all the other bits that are required.

Velvet worms are equally interesting. These are sometimes called “living fossils” because they have existed, almost unchanged, for some 570 million years. These caterpillar-like creatures are about five centimetres long and resemble arthropods in having many pairs of legs, but their bodies are not segmented and so they cannot be classified as arthropods. Their bodies are covered with microscopic projections that give them a velvet-like texture; hence, their common name. My QEP species belongs to the genus Peripatopsis and our two local species can be identified on their behaviour when disturbed. One species curls up like a millipede while the other spits at you. So the one I found is P. moseleyi because it spat at me. Velvet worms are predators and when they encounter suitable prey they squirt a sticky white fluid all over it. This glue sets quickly, and effectively immobilises the prey. The worm then bites a hole in its prey, injects digestive juices into it, waits for the juices to digest the prey’s cells and then sucks up its predigested meal. Velvet worms are a step up in evolutionary terms from the flatworms in that they breathe through fine tubes (tracheae) which feed all parts of their body with oxygen.

Space does not allow me to elaborate further so why not Google flatworms and velvet worms, and learn more?

• Dr Jason Londt is a natural scientist with a special interest in entomology. He welcomes queries and comments, which can be sent to him at jason

londt@telkomsa.net. Please do not send large attachments.

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