I have created a few creatures with a pattern based on that of the earthworm (head and foremost back-portion is dark, belly and rear is paler). Both of these creatures have eyes, and on one creature the pattern is on the hair, not the skin. However, I have recently discovered that the earthworm's distinctive pattern is actually used to detect light without eyes, which is not a necessary feature when the creature actually has eyes. What other reason could there be for this pattern to arise?
What is the Motivation?
To answer this question, we need to understand a lot more about what motivates the creatures with this body plan + Eyes. You don't tell us anything about the individual species, what they eat, where they live, or what eats them. I'll assume you want us to create species that look like this. The reasons are as diverse as the motives and driving forces of a species (and yes, impressing the opposite sex is ALWAYS a possible motive). First, let's assume something like an earthworm.
- So our organism is the pseudo-worm, a parasitic organism with a life cycle that has one phase in the soil. Animals see an 'earthworm' and come down to eat it. The pseudo-worm is swallowed whole by things that eat earthworms. But at that point, the critter starts doing all the grisly things to the animal that 'ate' it that parasites can do. Have fun! It can drill a hole in a bird's skull and take control, making the bird first seek out an opposite-sex pseudo-worm to eat, then spend weeks shedding baby psuedo-worms into the environment.
- OR earthworms in your world are poisonous to many species, except for a few killer snails. Despite the dull appearance, many predators avoid earthworms out of fear. The vision of your species means that it can outmaneuver the snails that can digest the poisonous worms, but still not get eaten by the fast things that are smart enough not to eat poison worms.
- For things that aren't like earth worms, mimicry may allow an animal to infiltrate a herd for protection. I know a certain sheep dog that looks just like the sheep he works with, and the sheep seem to almost accept him as a member (although I personally don't find sheep terribly bright). A symbiotic relationship may depend on inter-species appearance and communication based on visual signals important to the other species.
Pack communication & tactics:
Now lets assume a pack/herd animal.
Your animals signal submission to the pack with light fur. So females avoid aggression during mating when they present their back ends, and any member of the pack submits by rolling over and showing their light belly. Dark heads signal aggression and dominance, and are presented to assert rank.
When hunting, a predator wants to be undetectable to its prey, but recognizable to its pack. The dark, camouflaged face is towards the prey, while the visible hind (and belly, for prone pack mates and those in burrows) is presented to the pack.
A prey herd animal might have the opposite strategy. The visible face is used to draw the attention of predators to females defending young or large males defending the herd, while the camouflaged light belly and hind quarters are presented to the predator chasing them.
Give me a set of driving forces, and we can select a specific cause for your species.
Adaptation to predation.
The "earthworm-a-likes" are adapted to survive predation from above (voles, birds even the occasional fox). The knack to this is to look dark against the dark soil beneath them (it presupposes that they spend time on the surface during the day - perhaps like real earthworms when it rains, they come to the surface to avoid drowning.)
Melanosomes (what makes dark-skin dark and gives you a sun-tan) in skin cells have the special knack of reducing skin cancer by absorbing harmful ultraviolet radiation from your Sun. This means that in response to light from above there would be an adaptive advantage to those that get the dark pigment on their dorsal sides, their backs.
There's simply no need to have any dark pigment on the underbelly (the ventral side), and the stimulation of sunlight to grow the pigment would be absent.
Another adaptation to favour a light belly might be a tendency to swim in (or cross) fish-infested-waters - the light belly against the sky would be less conspicuous visually (when seen from the underneath against the light sky), and be less likely to attract the attention of the hungry ones below, and conversely a dark back against the water would be less visible to hungry flying creatures. You see this in most shallow-water fish.
The adaptation to having eyes is so ubiquitous, so useful in mate-finding, feeding, finding shelter, avoidance of (or freezing to be less visible to) predator animals, that yes, you can absolutely justify its existence.
For segmentation like worms, there are all sorts of insect larvae which, like caterpillars, have those periodic bulges, and move with rhythmic muscular contractions. Their eyes are a bit different, but they work just fine. The Leatherjacket (crane-fly larva) is a nice example (although a bit stumpy and fat compared to worms), living underground much of the time, but they appear sometimes when it rains (their eyesight is pretty awful, but they can detect light just fine).
For another reference, you can take a look at the slow-worm - it looks a bit like a snake, dark on the back and light belly, but it's a reptile in a separate group from snakes.
Now for the lighter tail there are several possible explanations for the evolutionary "usefulness" of this adaptation:
The species with this pattern signals to predators that it is nasty or deadly to eat. An example of this on Earth would be the Yellow-banded Poison-dart frog, but in the case of your creature, the absence of melanocytes in the tail might be accompanied by a reflective inner layer, not of visible light so much, but of ultraviolet perhaps - this supposes that your equivalent predator species has eyes that see in that part of the spectrum as bees and other creatures can.
Many lizards and some spiders, crabs and creatures such as starfish practice autotomy, the ability to detach a body-part when attacked to allow the creature to escape being eaten. In the case of some lizards the tail wiggles frantically for some time to keep the predator busy trying to eat it. All the creatures mentioned can regrow that body-part to decoy again. Under decoy, there's another interesting category of mimicry: many creatures, amphibians, insects etc. have evolved to look like another species which is poisonous, this deters predators.
Some spiders, deep-sea fish,, insects and snakes use body-pars as a means of attracting their dinner to approach. The Akistrodon, a type of snake related to the pit-viper is a bit slow-moving, so like it's meals delivered. It has a light coloured tail, which it wiggles invitingly to look like a delicious worm or grub, this attracts the interest of small rodents, and amphibians which would normally feed on such treats. Once in range they're easily snapped-up and devoured by the snake. Strangley, there is a creature that uses sexual allure to attract it's meals too: the Photuris firefly has a dirty trick, it mimics the female flashing signals of other fireflys, attracting males which then promptly get devoured. You didn't ask about a glowing tail, but I thought I'd place the option there anyhow.
Now, peafowl have an obvious display in the males. The peacock's massive colourfull tail which they spread and jiggle at the females in a sort of dance is tremendously costly in terms of the male's resources, it slows them down, makes them more likely to get eaten. It's a real effort for the male to survive and grow such a tail to get to the point of reproduction - it is therefore seen as a sign of strength vigor, fitness and generally very attractive to females. Seems paradoxical, but it works for them. Your creature, under danger of aerial attack, would seem foolish to grow a conspicuous light-coloured tail, but maybe such a bold strategy is attractive to the female of the species. (Or vice-versa re. the sexes if you wish).
Sometimes people just want a pet that has the traits of an earthworm.
The smooth fox terrier is a dog breed that is white, with a dark head and optionally some large dark spots on the neck or torso. Sometimes they don't have those spots, so they are effectively what you are looking for.