Looking rather like an alien from a Star Trek episode, this sea butterfly snail is just one of many marine animals found in a photo gallery of translucent marine life on the National Geographic website. However the strangest cartoon-like fish has to be the Pacific barreleye shown below in an image from the National Geographic website (from the Monterey Bay Aquarium Research Institute).
In an effort to study the internal organs of live fish, researchers in Japan actually developed a translucent goldfish last year. I bet these will be popular models among Comparative Physiologists! You can watch a report about these fish here.
Finally, in my search for interesting translucent animals, I came across this fun You Tube photo series on the "top 10 transparent animals". Despite the typos in the text, it is pretty neat to see the wonderful array of animals.
When I first saw the video of the Pacific barreleye I thought that it was animated, it justs looks too strange. And I still think that it is one of weirdest and most interesting animals on our planet :)