Is the venom a poison? If so, does it need the bit about delay and neutralize poison? (Just bookkeeping here.) But it looks pretty good.
CR 1?
It's a poison, I only left the bit about spells in for the sake of explicitness, feel free to remove it if you think it's completely superfluous.
I guess I should have said "poison" in its description somewhere, just stick it in at the start:
Agonizing Venom: Poison, DC 15 Fortitude negates, initial damage wracking pains (imposing a -4 penalty on attack rolls, skill checks and ability checks for a duration of 1d6 ×10 minutes), secondary damage 1d6 Dexterity. Agonizing venom can be suppressed by the delay poison spell or neutralized by the remove poison spell, or similar effects. The save DC is Constitution-based and includes a +4 racial bonus.
Now I've been wondering whether we should reduce the bristle damage. Are these piercing spines or more like stinging hairs? If the latter, they may only do 1 point of damage or none at all. Checking the original description again, they do 1d2 damage, shall we change it to that?
Hmm, does that mean that I've undervalued its bite attack? A 1d4 bite seems more appropriate for a Large creature.
CR1 is about right, I guess. It's not much of a threat apart from its agonizing venom. A Eurythoe could be nasty when combined with monsters that are immune to its SA - imagine a sunken ship crawling with eurythoe and low-level undead.
A couple of points about the rest of the stats.
I don't believe Eurythoe have extending "necks", so it probably only has a 5 foot reach.
The Advancement is probably our standard for worms.
Advancement: 3-4 HD (Large); 5-8 HD (Huge)
According to my rough reckoning length and weight is something like.
A typical mature eurythoe is 2 to 4 feet in diameter and 5 to 7 feet long, weighing between 300 and 1500 pounds.
That's assuming it scales evenly, a 2' diameter and 5' long specimen comes out as under the regular lower weight for Large but there's naught wrong with that, some Large SRD animals are under 500 pounds too (e.g. lions).
Oh, and its full attack line has a different bite damage to its attack line. Let's pick one and change the other. I'm thinking 1d4 bite now, as mentioned earlier.
