(Only technically they aren't four fins:
They all share a very distinctive paralarva known as the doratopsis stage . . . Doratopsis paralarvae and the adult stages of some species have a gladius that extends well posterior to the fins and supports a remarkable "tail" that bears a variety of ornamentation, mostly of uncertain function. The ornamentation may be in the form of an oval structure superficially resembling a pair of fins, or a series of small flaps and/or oval bulbs.)I got to know chiroteuthid juveniles pretty well while sorting plankton this summer, so it was with a sense of affectionate familiarity that I read about the recent discovery of a dead chiroteuthid floating on the sea surface off the coast of Grand Cayman.
I had never heard of this particular species, but, thankfully, it bears a descriptive Latin name: Asperoteuthis acanthoderma, which means "rough squid spiny skin." It's a bit redundant, but we get the point: this species is covered with sharp little tubercles. And A. acanthoderma gets big--the largest reported was 78 cm in mantle length, and the tentacles can supposedly be twelve times as long as the mantle.
You know, now that I'm thinking about it, I have a hard time imagining cranchiids (also called glass squid) calling one of these beasts anything at all. They probably just keep quiet and give it a very wide berth.