No need for evasion on the Nereis, I think.
Agreed to improved grab and chew for diopatra. The parapodia might just give a bonus to grapple checks, or they could also constrict along with chewing.
Good, enworld is up and running again!
I think the Nereis is done with unless we change our minds on the 'Escape Response'. I couldn't see anything to niggle about apart from the weight.
As for the Diopatra, I agree with the Improved Grab/Chew mechanism. We could just swap some nouns in the octopus's version of the ability:
Improved Grab (Ex)
To use this ability, an octopus must hit an opponent of any size with its arms attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and automatically deals bite damage.
Improved Grab (Ex)
To use this ability, a diopatra must hit an opponent of any size with its bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity, with a +4[?] racial bonus to its grapple check. If it wins the grapple check, it establishes a hold and automatically deals bite damage.
Oh, and I think we need a lunge attack, something like:
Lunge (Ex)
A Diopatra can lunge out of its burrow as a full-round action, stretching up to twice its reach (10 feet, 20 feet for Huge worms, 30 ft. for Gargantuan) and then making a bite attack with a +4 circumstance bonus. This counts as a charge.
I'm starting to reconsider the Escape Response, it may be worth giving to the Nereis and Diopatra. Something like:
Escape Response (Ex)
A Nereis can duck inside its burrow with lightning speed to avoid attacks. This is an immediate action. The attacker must succeed on a DC15 Reflex save to hit a Nereis using its escape response with a weapon attack.
A Nereis retreats whenever it is threatened by a creature with a size category larger than itself. This is an automatic reflex over which the Nereis has no control, but it can also trigger its escape response whenever it chooses.
To use its Escape Response ability, the Nereis must have part of its body within its burrow. The worm automatically releases any opponent held in its jaws if its escape response is triggered.
After triggering its escape response, the nereis will have moved 10 feet inside its tunnel. Thus, it must make a burrow move action to return to the entrance of its lair before it can attack any creatures outside.
The saving throw DC check is Dexterity based and includes a +4 racial bonus.
The Diopatra presumably could have the same ability, since its a burrow dweller as well.
If we do give the Nereis this Special Ability I think it should bump its CR up a bit, since it considerably improves its defenses.
EDIT: How about we keep the Nereis as is and just give Diopatra the Escape Response, it's got the ability to lunge out of its burrow with lightning speed, so having it be able to retreat with similar alacrity has a pleasing symmetry.