I notice you have two similar posts. I have answered the other one, and with it I answered this one.
But to elaborate, this is the abdomen or tail of a female, or hen crab. The outside surface appears as a row of plates, which hinge and lay tucked underneath the crab. Underneath this there are four pairs of pleopods which appear as feathery "legs" under the tail. It is on these feathery pleopods that the female crab will carry her eggs when "in berry". To accomodate all this, the abdomen is significantly wider in the female than in the male.
The abdomen of a male, or cock crab, on the other hand, is narrow and appears as an elongated triangle underneath the crab. As in the female there are structures underneath the tail which facilitate reproduction. In the male, there is only one pair of pleopods, quite different to the female.It is these two "legs" which are used in conjunction with the two female genital openings as in your other photo.