That would be a gelatinous cube. Not exactly the same thing, though both are vulnerable to fire and acid.
Cubozoans are what are commonly called box jellyfish. They are superficially similar to true jellyfish but the two groups are about as closely related as humans are to eels. The cubozoans are more advanced than jellyfish and are moderately active predators, and they usually possess several imaging eyes to enable them to locate their prey.
It depends a lot on who you ask, and I freely admit my knowledge is about 15 years old so things have probably changed.
Basically there are chelicerates, which have a fairly bizarre embryology and a fairly simple body plan where the mouthparts are essentially just modified legs located at the front of the body. These include the spiders, ticks, mites, horseshoe crabs and so forth.
Then there are the Uniramia. These have the same basic embryology as the annelids and onycophorans but the mouthparts are heavily modified to the point where they no longer resemble limbs at all, and the mouthparts aren’t located on the front head at all, but well back behind the antennae. This group includes the insects, centipedes, millipedes and so forth.
Thirdly there are the Biramia, comprised mainly of crustaceans with a couple of odd side-groups. These show an initially similar embryology to the unirami and annelids and then run off on a bizarre tangent. They also have heavily modified mouthparts that are positioned behind the antennae, but the mouthparts and all the other appendages split close to where they leave the body resulting in an inner and an outer limb.
The exact evolutionary relationships of these groups is up for debate. Some argue that the three groups all evolved independently form annelids (or even more primitive ancestors in the case of the chelicerates). Some argue that the chelicerates gave rise to the biramia while the uniramia evolved separately directly from annelids. Yet others argue that the biramia gave rise to the uniramia.
The best evidence AFAIK is that what we call athropods aren’t very closely related to each other at all, and that the superficial common features are all the result of convergent evolution dictated by the independent evolution of an exoskeleton. There are only a limited number of ways to breathe with an exoskeleton, and only a limited number of ways to see, only a limited number of ways to grow. As a result the groups have independently evolved compound eyes, spiracles, moulting and so forth. That tends to be borne out when the details of those features are examined. Although the end product is the same the fundamental development is very different on all groups.