Earthworm anatomy varies considerably and is complex.
One factor that makes earthworms very difficult to identify is that some species are homomorphic. That is to say, where there are dense concentrations of mixed species, the instinct for survival, or more simply, the need to compete more successfully for food, provides some smaller species with the ability to rapidly increase their body size to approaching that of the larger.
Certainly, quite apart from this, worms have the capacity to diminish or increase in size with amazing rapidity according to the supply of food.
Usually the most obvious feature of an earthworm is its clitellurn, the section of the body which denotes sexual maturity. This is a saddle or ring, sometimes raised or wider than the body, sometimes not. Its position varies between species and its position and shape is used as one of the first steps to identification.
On each segment along the worm’s body are horn-like hairs called setae, which are used for grip (and also as sensors). These are arranged on the body in differing patterns or groups according to the species. The pattern is another surface feature used in the identification of worms.
An earthworm’s circulatory system is controlled by three to five pairs of simple hearts with the positioning and number varying between species. These provide one of the reasons why the old story that, if you cut a worm in half, you have two worms, is largely wrong. With a few exceptions, worms need all their hearts and, if you cut some off, they will die. Generally, it is possible only to cut a worm close to the tail if it is to survive.
Some species — the Blue (Perionyx excavalus) is a good example — have remarkable regenerative capabilities. On exposure to light they will flip themselves about so vigorously that they can break in half. Blues are often able to survive and rebuild themselves under these circumstances, although sometimes they end up a lot shorter than before they indulged in such violent gymnastics!
Apparently, some worms have the secret of eternal life. A Red worm kept isolated for fifteen years was killed and dissected. No signs of ageing were found. But this is not true for all species, some of which are thought to live for a year only, while others have been found to have a two-year cycle.
Worms need a similar supply of oxygen, weight for weight, as we do, but they are able to extract their oxygen efficiently from what to us would be a very uncomfortable environment, like soil_ This is because their blood haemoglobin has a much greater affinity for oxygen than ours and, conversely, can cope with much lower oxygen levels. They breathe, or absorb oxygen, through their skins.
Knowing this, it is easy to understand why it is that after heavy and prolonged rain, particularly in the mornings, worms can often be found dead, scattered about on the soil surface. The reason is simply that as the soil becomes saturated with water, the air, being less dense than the water, is displaced and the worms rapidly begin to suffocate. They are, therefore, forced to the surface. The reason why they die is not fully understood, but it seems that perhaps they postpone this surfacing until it is too late, arc unable to recover from their near-suffocation, and die. If this surfacing occurs in daylight, then exposure to ultraviolet light causes their deaths,