What type of muscle surrounds the pupil causing it to constrict?

The iris is the colored circular structure surrounding the center of the eye, the pupil, which appears to be black. From Wikipedea I quote: "The size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilater and sphincter muscles."

The sphincter muscle (or pupillary constrictor) is in the part of the eye called the iris, and encircles the pupil, doing its part to constrict the pupil during accommodation to bright light. Short ciliary fibres innervate the sphincter muscle, run through the sclera and choroid, and travel through other structures, including the Edinger Westphal nucleus, to be ultimately controlled by parasympathetic fibres within the parasympathetic nervous system.

The iris dilater muscle runs radially in the iris in a spokelike fashion of contractile cells; when stimulated, these widen the pupil to allow more light to enter.

This parasympathetic response to light is usually only applicable to humans- the lower animals don't have brains that are involved in the process, instead their eyes are photosensitive.