R-Type changed shoot-em-ups forever. It took the core gameplay of Scramble and Gradius - fly from left to right, collect power ups and kill things - and blasted the genre into new, previously unimaginable realms.

After R-Type's release to arcades in 1987, developers had to come up with their own weapon systems to match the innovation and sheer cunning of that game's Force, an indestructible orb that could be used as a shield or a brutal battering ram to tear into the enemy's ranks. After R-Type, every shooter needed a spectacle to match the curling mass of tails and eyeballs that was Dobkerratops, the creature that scrolled menacingly into view at the end of the first stage, or the bravura spaceship found in stage three, a monstrosity so vast that it had to be destroyed one piece at a time - in essence, it was one relentless boss fight. In short, R-Type became the standard by which all other shooters were judged.

Indeed, R-Type cast a shadow so great that Irem never quite stepped out of it. They released many other games - shooter or otherwise - but these never reached the creative pinnacle that their most famous creation represented. Like Joseph Heller with his most famous novel, Catch 22, or Orson Wells and his directorial debut Citizen Kane, Irem's name became synonymous with R-Type, and everything that came after it - including the R-Type sequels themselves - were deemed anaemic by comparison.

X-Multiply was one of three shooters released by Irem in 1989, and in many ways it's simply a variation on a theme; another side scrolling shoot-em-up with bio-mechanical visuals, there's initially little to suggest that X-Multiply is anything more than a derivative regurgitation of the developer's previous ideas, its premise - a miniaturised ship injected into a human body to destroy alien parasites - a flimsy excuse for an extended reworking of its illustrious predecessor's visceral second stage.

Only gradually does it become apparent that X-Multiply does, in fact, have an identity all its own; the weapon system, which initially feels like a cagey re-working of R-Type's force, eventually becomes a deadly and eerily graceful tool of destruction. Where other late eighties/early nineties shooters were all too keen to head down a blind alley of flashier ordnance to an oddly desensitising effect, Irem wisely took another, more austere path.

X-Multiply's ship - an oddly anonymous metallic dart when compared to the R9's distinctive blue dome - featured a weapon that resembled a pair of floating tentacles; these whipped and curled around the front or rear of the craft depending on the direction of movement, providing brief protection from incoming fire while spitting lasers from their tips.

It's a system that requires patience - and numerous credits - to master, a process not helped by X Multiply's vicious difficulty level, which hurls bullets, enemies and craggy outcroppings at the player from the off. But at the same time, it's a system that rewards perseverance, and in time proves to be an efficient and elegant form of defense. With experience, using the tentacles as deadly whips to swipe and destroy enemies becomes an efficient kill method, and it's possible to play through whole sections of the game without firing a single shot - a unique concept that would inspire later (and inferior) shooters like Jaleco's St Dragon.

And while X-Multiply's visuals owe a clear debt to HR Giger, there's no denying the gruesome imagination evident in most of the game's sprites. On its arcade flyer, Irem described X-Multiply as a 'horror shooting game', and so it is:  an almost ceaseless barrage of tentacles, veins and reptilian xenomorphs. Level two introduces a colossal, level-spanning ship even more nightmarish than the one found in R-Type, a chimerical fish/abomination that takes considerable skill to destroy.

But despite the wealth of great ideas found in X-Multiply, it failed to generate a tenth of the devotion that R-Type enjoyed. While dozens of derivatve R-Type clones were converted to computers and consoles throughout the late eighties and early nineties, X-Multiply vanished without trace, bundled with Irem's other minor league classic Image Fight for Sega Saturn and PS1 in 1998 as an Arcade Gears collection.

So why did a shooter of X-Multiply's calibre fail to capture the public's imagination? Was it the brutal difficulty level, or its initially opaque weapon system that took time to reveal its hidden depths? These are both plausible theories, but the answer, I'd suggest, is that X-Multiply was simply lost in the tidal wave of shooters that hit the arcades in the late eighties; flanked on either side by cabinet after cabinet of derivative alien massacres, and eclipsed by R-Type II released earlier the same year, X-Multiply failed to find the audience it deserved.

The author Joseph Heller, who despite a lifetime of novel writing will always be remembered for his debut work, once said: "When I read something saying I've not done anything as good as Catch-22 I'm tempted to reply, ‘Who has?'"

In the numerous shooters that Irem churned out in the years after 1987, X-Multiply came the closest to recapturing R-Type's pioneering spirit, and ranks as one of the genre's great forgotten masterpieces.

Ryan writes his gaming column every week at Den Of Geek. Last week's is here.