
All Sliver creatures get +1/+1.
The air was filled with the cracks and snaps of flesh hardening as the new sliver joined the battle.
Illustrated by Richard Kane Ferguson
Released on October 14, 1997
Muscle Sliver's ability does give itself +1/+1.
Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a Sliver may become lethal if Muscle Sliver leaves the battlefield during that turn.
Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn't provide any additional benefit.
If the creature type of a Sliver changes so it's no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures.