Due to inconsistencies in the design, the skeleton looks differently every time it appears. However, most of the time it is a white and gray skeleton with SpongeBob's similar eyes and teeth. The skull is always seen as rectangular, which supports SpongeBob's skin to become solid.
Role in series
"Mermaid Man and Barnacle Boy III"
Whem Man Ray is free from the tickle belt, he uses some of Mermaid Man and Barnacle Boy's gadgets to zap SpongeBob and Patrick and their skeletons appear.
SpongeBob says to Squidward that they will both work at the Krusty Krab forever. Squidward imagines what this would look like. SpongeBob and Squidward are then shown in skeleton form with SpongeBob's skeleton behind the cash register.
When SpongeBob finally gets scared by the Flying Dutchman, he splits up into his skin and his skeleton. The two run away together from the Flying Dutchman's ship until they make it back to Bikini Bottom, where they merge back into normal SpongeBob again.
SpongeBob opens his face for his everyday items. By doing so, he reveals his skeleton.
Role in other media
The skeleton appears in the book Bare Bones as part of the main story where, after successfully scaring SpongeBob, Patrick and the sponge chases the runaway skeleton that jumped out of SpongeBob.
The skeleton appears as merely cameos in the video games, most of the time as for comedic purposes. In the console versions of SpongeBob's Truth or Square, the skeleton appears as the final idle animation that plays if the player idles for too long, which has SpongeBob aging quickly to his death. The bones will fall apart and quickly turns into the regular SpongeBob if the player makes the sponge move.
The existence of the skeleton inside of SpongeBob, like the appearance, has been inconsistent throughout the series. Some episodes or media depicts SpongeBob not having any skeleton from the inside aside from organs.
In various episodes, SpongeBob's interior is depicted to be completed empty, aside from the mouth, eyes, and pores, the biggest example of this being "Slimy Dancing."
In most appearances, the skeleton is depicted to have its own sentience separate to SpongeBob. Although, most of the time, it is for comedic purposes only.
Biologically, SpongeBob should not even have a skeleton since sponges are invertebrates.