Very nice catch TRG!! That is generally how animation was done in the 2D days. First the main animator (or key animator) would do the rough drawings of the scene, maybe polish them up a bit. Then these would go to his/her assistant or the clean-up artists who would carefully trace over the image and produce a clean final drawing which would then go on to be Xeroxed onto clear acetate sheet (or cel as we commonly call them) and then colored on the back and finally photographed by the camera to make one frame of the cartoon. Tough work! Sometimes they would do only partial drawings of a character, like their mouth or head only when the character is talking but the rest of his/her body is not moving. No point in re-drawing the entire body when it is static. Wastes time and art supplies! I think that Slithe face drawing might be one of those.