Pathfinder SRD > Classes > Base Classes > Summoners > Eidolons wrote:
Grab (Ex)
An eidolon becomes adept at grappling foes, gaining the grab ability. Pick bite, claw, pincers, slam, tail slap, or tentacle attacks. Whenever the eidolon makes a successful attack of the selected type, it can attempt a free combat maneuver check. If successful, the eidolon grapples the target. This ability only works on creatures of a size one category smaller than the eidolon or smaller. Eidolons with this evolution receive a +4 bonus on CMB checks made to grapple.
When you bite (if you chose bite), you immediately get a free try to grapple.
Pathfinder SRD > Classes > Base Classes > Summoners > Eidolons wrote:
Constrict (Ex)
An eidolon gains powerful muscles that allow it to crush those it grapples. Whenever the eidolon successfully grapples a foe using the grab evolution, it deals additional damage equal to the amount of damage dealt by the attack used by the grab evolution. This evolution is only available to eidolons of the serpentine base form.
If that grapple granted to your bite by Grab works, you also immediately do constrict damage (the damage is equal to your bite).
Pathfinder SRD > Gamemastering > Combat > Combat Maneuvers > Grapple wrote:
Grapple
As a standard action, you can attempt to grapple a foe, hindering his combat options. If you do not have Improved Grapple, grab, or a similar ability, attempting to grapple a foe provokes an attack of opportunity from the target of your maneuver. Humanoid creatures without two free hands attempting to grapple a foe take a –4 penalty on the combat maneuver roll. If successful, both you and the target gain the grappled condition. If you successfully grapple a creature that is not adjacent to you, move that creature to an adjacent open space (if no space is available, your grapple fails). Although both creatures have the grappled condition, you can, as the creature that initiated the grapple, release the grapple as a free action, removing the condition from both you and the target. If you do not release the grapple, you must continue to make a check each round, as a standard action, to maintain the hold. If your target does not break the grapple, you get a +5 circumstance bonus on grapple checks made against the same target in subsequent rounds. Once you are grappling an opponent, a successful check allows you to continue grappling the foe, and also allows you to perform one of the following actions (as part of the standard action spent to maintain the grapple).
If you continue the grapple granted to your bite by Grab and that grapple is successful, you also immediately do constrict damage (the damage is equal to your bite).