OTHER SOIL ANIMALS - The Soil - page 106

Earthworms continue their fascination for scientists in many different ways;
about 2500 papers on earthworms were published between 1930 and 1981. Darwin's
topic on earthworms was published in 1881 so, in 1981, a Darwin centenary symposi-
um was devoted to earthworm ecology with contributions from 20 different countries
- from the Philippines to Canada and the USSR. Perhaps the most recent theme of
interest in a hungry world is vermiculture, to see whether worms can be cultured on
various waste materials to provide food for fish, livestock, or even direct human con-
sumption. Not only do worms have a high protein content, but analyses show that this
protein is rich in essential amino acids.
S OIL NEMATODES
Parasitic nematodes are associated with most kinds of plants and animals; man alone
is host to some 50 species which, as threadworms, roundworms or hookworms, are
perhaps better known than the more unobtrusive soil dwelling forms known as eel-
worms. Nematodes are omnipresent in soils just as they abound in the bottom ooze
of oceans, lakes and rivers. Next to protozoa, they are, indeed, the most abundant soil
animals, with populations up to 10 or 20 and even 30 million per square metre in the
top few centimetres of pasture and woodland soils. Here they form a major compon-
ent of the interstitial micro-fauna along with protozoa, rotifers, gastrotrichs and tardi-
grades, crawling or swimming in films of water through the labyrinth of pores.
The term eelworm is an apt description of these tiny creatures, mostly 0.5-2mm
in length by 0.02-0. 1mm in breadth, as they thrash back and forth in a characteristic-
ally serpentine manner in a dish of water. Like eels and earthworms, nematodes have
a definite dorsal and ventral surface but they have a much simpler musculature: they
cannot expand and contract like earthworms, nor can they bend from side to side as
they appear to do. Their movements are almost entirely confined to the dorso-ventral
plane - as in traditional pictures of the Loch Ness monster; because of this, they are
forced to lie on their sides when wriggling on a flat plate ( Fig. 36 ) .
There are perhaps one to two hundred species of eelworms that one might en-
counter in an uncultivated soil, but 95 per cent of these are likely to belong to ten
or a dozen common species while the rest are relatively rare. Most free-living forms
are very alike - vermiform, smooth-skinned and transparent - and it requires a good
microscope, and much practice, to discern the characters that distinguish them. The
anus is not terminal, as it is in earthworms, so there is a true tail (as in eels) whose
form and relative length are useful in identification. However, the mouthparts provide
some of the most important features as they immediately give clues to the method of
feeding.