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You can polymorph into a drider or Tiny, Small, Medium-size, or Large monstrous spider (see the Monster Manual). You regain lost hit points as if you rested for a day on the initial transformation. The spell lasts until you decide to resume your normal shape.
You acquire the physical and natural abilities of the creature you polymorph into, including natural size, Strength, Dexterity, and Constitution scores, armor, attack routines, and movement capabilities.
Unlike polymorph self, you acquire the poisonous bite and web-spinning ability of whichever spider form you choose. You do not suffer any disorientation.
You retain your Intelligence, Wisdom, and Charisma scores, level and class, hit points (despite any change in Constitution score), alignment, base attack bonus, and base saves. (New Strength, Dexterity, and Constitution scores may affect final attack and save bonuses.) You can cast spells and use magic items if you choose drider form, but no other spider form is capable of spellcasting or manipulating devices.
If you choose drider form, you equipment remains and continues to funciton as long as it fits the drider's body shape. Retained items include anything worn on the upper body (head, neck, shoulders, hands, arms, and waist). Otherwise your equipment is subsumed into the new form and ceases to function for the duration of the spell.