Translucent for who?
I.e. they may be translucent for human eyes but, if they use vision, very likely they won't be translucent for the members of the same species.
Translucency will require:
weak absorption of radiation with wavelengths the receiver (watcher) uses. I.e. some plastic may be opaque in VIS, but translucent/transparent in IR
refraction index almost match between the biological materials in the internal organs and between the biological materials and outer medium - otherwise the light will get distorted by some many surface separations one can no longer speak of translucency.
A leaf from the "Biology is just applied chemistry and chemistry is just applied physics" book - for the biochemical reactions to proceed, the catalysts (proteins mainly) will need to perform selectively activate the energy of the reaction centers in the (complex) biological molecules to make the controlled reactions (required by synthesis of complex building block) possible.
Light absorption: For Earth biology, it means energies in the 0.1eV - 4eV range. Well, guess what? That's the range in which the vision system of the Earth animal receives. And that should not come as a surprise: the vision receptors just use the (bio)chemistry of what is available in the most energy-dense available in the solar light spectrum at the Earth surface (UV and higher are strongly absorbed in the upper layers of the atmosphere, IR is too low an energy to support fast enough - for Earth-like terms - biochemistry).
To absorb less light (and appear translucent to a "watcher"), the organism will need to do one or more of the followings:
- low level of biochemistry required - e.g. being a parasite that takes most of the substances from the host and just recombine them using biochemistry at lower energies
- use a biochemistry tuned to other sources of energy than the light and do it at energy levels not used by the "watcher" - IR (which means slower reaction rates at the same ambient temperature) or UV+ (e.g. a biology using radioactivity as the energy source - such organisms would likely appear not only translucent but also luminescent to the human eyes)
- use the same biochemistry as the "watcher", but in very diluted centers - should be physically huge (to allow superior biological functions at low concentrations) and likely slow in human terms. Doesn't mean they couldn't manipulate huge amount of energies if they reach planet-size range, but don't expect them to be humanoid size entities.
Light refraction well, there may be some trouble here:
- mostly, this is achieved by using the medium they live in for the most percentage of their body makeup - in your jelly-fish example, mostly water...
- ... but then changing the medium they live and evolve will likely create some difficulties - like evolving a tougher and less permeable skin. Unlikely it will be transparent, though not impossible.
Well, in regards with they won't appear translucent to themselves - very likely they'll need to use some form of social groups large enough, be it only because "selfish intelligence" is so less efficient than the sharing of knowledge and cooperation. So, they'll need to perceive themselves somehow. The only way they can do it and remain translucent is to not primarily use vision in their interaction and thus make their "visible appearance" a trait with no evolutionary advantages.
So, Ok, maybe I don't answer to "how they evolved" but I hope this answer is giving you some points to build on top. Feel free to poke holes in my considerations anyway, I'm not providing any warranties of "absolute truth" with them.