McDonald's Happy Meal web site confirms that the October 2015 kids' meal theme will be based on the current film Hotel Transylvania 2 (not The Nightmare Before Christmas, which was released back in in 1993).
There are differences in the shell textures of oysters and clams. The shell of a clam is smooth and shiny while an oyster has a rough outer shell. Both the organisms take in water through the shells but the filtering procedure is different. Oysters produce pearls which are in much demand but clams do not.
It's hard to imagine that a clam could be a close cousin to the earthworm, because most familiar molluscs have a highly modified body type. The ancestral mollusc probably resembled a chiton, a flattened worm like animal protected by a dorsal shell. Both molluscs and annelids probably evolved from free-living flatworms.