Terrestrial Flatworms

By Andrew Fraser

"Flatworm" is the name normally used for the group of invertebrates called Planarians, an order of the phylum Platyhelminthes. Unlike the true worms, flatworms are not segmented and do not have a linear gut with a mouth and anus. Instead they have a gut with a single opening in the centre of their ventral surface from which they are able to extrude the outer part of the gut to engulf and digest their food externally. Their food is a range of small animals including snails, worms and other invertebrates. As the name suggests, flatworms are flattened dorso-ventrally and have a great capacity to change shape as they can stretch out or contract considerably. They move by a gliding motion using slight muscular contractions aided by cilia on their ventral surface, which move over a thin mucous layer they lay down.

In Britain there are about 29 species of flatworms, of which 23 species are found in freshwater, both ponds and flowing water. Some of these are widespread and can be abundant, others are very restricted in distribution. However, six species can be found living in the soil or amongst leaf litter, of which only two or possibly three are native to Britain. On land, they move through the soil or leaves following cracks or the tunnels of other animals such as earthworms. As they are very vulnerable to desiccation, they are normally found in damp places such as under stones or logs. As the ground dries in summer they move deeper to avoid drying out.

In Worcestershire we have very few records of these terrestrial flatworms. I have found two species in my garden in Alvechurch - Microplanna terristris and Geoplanna sanguinea. The former species is a grey/brown flatworm growing to about 25 mms in length and 1.5-2.0 mms wide and more cylindrical than the usual aquatic flatworms. The second is an introduced species from Australia. It is up to about 35 mms long (80 mms when fully extended) and 4 mms wide. It is a cream colour with a slightly greyish or reddish anterior. Both feed on soil invertebrates such as slugs or worms. I have only found Microplanna in one other Worcestershire site, the Wildlife Trust's Broadway Gravel Pit Reserve. The Worcs BRC. has no other records of these or any other species of terrestrial flatworms.

If you are a keen gardener keep your eyes open for flatworms, especially when moving stones or in wet weather on the surface of the bare ground. If you find one, I would be interested in the record. As identification is difficult, it is probably best to preserve it in a small amount of, preferably white, vinegar and let me have it at the Trustís office so it can be checked. They need to be preserved quickly as they will rapidly die and auto-digest themselves.

Finally you may have heard of the dreaded New Zealand flatworm Artioposthia triangulata which is a voracious carnivore of earthworms. This has been accidentally introduced into parts of N. Ireland and Scotland and a few other places where it can cause problems in the soil by killing most earthworms. As yet it has not been found in Worcestershire, but keep your eyes open for a large flatworm, about 50 mms long and 10 mms wide, but which can extend to 150 mms. The colour is dark brown with a pale margin along the sides and a pink anterior tip.

If you are interested in further information on flatworms, the best book for their identification of this in the UK is British Planarians by I Ball and TB Reynoldson. It is published by Cambridge University Press as No 19 in the Synopses of the British Fauna series. ISBN 0 521 23272 1.

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