Earthworm Evolution
Earthworms, like all annelid worms, are metamerically segmented. Not only is the body of the worm divided
into a series of segments, but those segments are a repetition of one another, such that each segment
contains the same anatomical systems, more or less. Thus, each segment has its own muscle systems, its
own nerve centre, its own excretory organs (metanephridia), its own coelom and segmental blood vessels.
These systems repeat along the worm, with one set in each segment. Furthermore, each segment (except
the first few) is divided from adjacent segments by a dividing muscular membrane or septum. In this sense,
it is also as if each segment was an independent animal body and that the worm was a series of such
animals connected together. Indeed there are non-segmented worms (certain flatworms) that reproduce
by budding off a new individual at their rear. Sometimes the separation of the two new worms is delayed or
inhibited and a chain of connected worms results, each individual being physically connected to the
individuals in front and behind it, but each still possessing its own brain and other body systems. It has
been (convincingly) suggested that segmented worms evolved from such chains of worms that failed to
separate. Alternatively, the genetic code may have simply repeated its design several times over,
producing a worm born as a series of 'individuals' fused together. The end result is pretty much the same.

Naturally, evolution would not need to leave things there - with so many spare individuals hanging around
it would be easy to modify a few of them and see what useful designs may result. The first few segments of
the earthworm are more tightly fused together - the individual segmental 'brains' or ganglia have fused into
a single larger and more powerful brain. Now our worm has evolved a definite head by a process called
cephalisation. More generally, adjacent segments may fuse into distinct body parts or tagma, a process
called tagmatisation. This trend is weakly developed in the earthworm, though certain segments adopt
reproductive functions. However, in insects the trend continued, with segments fusing into head, thorax
and abdomen.

These processes of segmentation and tagmatisation (including cephalisation) demonstrate how Nature
can make a more complex organism from a simpler one by repeating or reiterating the basic pattern. How
about the first segment - the prototype worm (like a flatworm perhaps) evolve?

The development of an individual multicellular organism from a single cell (fertilised egg) offers
suggestions of an evolutionary path, not least because it shows how the simpler can become more
complex in a series of stages. The fertilised egg cell of an earthworm divides to become a hollow ball of
cells. This ball of cells is common to the development of many animals and is called a blastula.