Every inch of a sliver's body is a potential claw, appendage, or projectile.

Thorncaster Sliver

Sliver creatures you control have "Whenever this creature attacks, it deals 1 damage to any target."

Secret Lair Drop

Magic 2014

Sliver creatures you control have "Whenever this creature attacks, it deals 1 damage to target creature or player."

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 you change the creature type of a Sliver you control 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 you control.

If you control multiple Thorncaster Slivers and a Sliver you control attacks, each ability will trigger separately.