Repel vermin swarm?

[Callum]

When a vermin swarm (http://www.d20srd.org/srd/typesSubtypes.htm#swarmSubtype) meets a repel vermin (http://www.d20srd.org/srd/spells/repelVermin.htm) spell, do you consider the swarm's total HD when adjudicating how the spell will affect it? I can see it either way - the swarm description does refer to it having a "pool of HD".

[Starbuck_II]

When a vermin swarm (http://www.d20srd.org/srd/typesSubtypes.htm#swarmSubtype) meets a repel vermin (http://www.d20srd.org/srd/spells/repelVermin.htm) spell, do you consider the swarm's total HD when adjudicating how the spell will affect it? I can see it either way - the swarm description does refer to it having a "pool of HD".

Yes, I think (if you are asking what I am comprehending) example:
http://www.d20srd.org/srd/monsters/swarm.htm
Bat Swarms have 3d8 HD = 3 HD.

Even if each individual one would be less than 3, when they swarm, they are considered 3.

[Keld Denar]

Also, you often have contiguous swarms. Two centipede swarms together might be considered one entity, covering 2 10' squares. If you had enough HD to turn 4 HD worth of vermin, for example, you'd disperse half of the centipede swarm (1 10' square, which is equal to 3 HD worth over vermin, the extra 1 HD is because it isn't evenly divisable).

Basically, each 10' square of swarm is considered to be one "creature", even though it is made up of hundreds or thousands of creatures. Thats the easiest way to adjudicate it.