Pale Sea Devil

Over a long, long time, this creature formed the perfect weapon to shoot down its prey.
It initially developed a mechanism used to kick away small objects. At first this was mainly used as a defending mechanism.
But over time, this mechanism also proved useful to shoot small objects, such as stones. But precision was lacking since it neither could really see where it was shooting, nor was the weapon especially effective or accurate.

Over millions and millions of years, the eyes slowly moved towards the gun and finally grouped around the gun, giving it perfect vision to precisely shoot its prey. The gun itself got much more sophisticated as well and uses a high-pressure system to launch objects at a high velocity. These objects may be either a self produced, venomous arrows - or (in case of a shortage of such) small stones or other objects, which it is able to pick up with the claw-like end of the gun.

Since the creature is quite vulnerable with the exception of the gun, it formed a hard shell. The tentacles on its left side act as antennas able to sense approaching danger.
In case it feels threatened, it will quickly disappear inside its shell.
Normally this creature is found in underwater caves where it also hides during the night. Usually this cave is filled with small objects which can be used as material for its weapon in case of an emergency.