Why are the mouth parts located on the ventral part of the crayfish

1. It makes it easy for the animal to take a bite out of something that's "below" it. Watch a crayfish that's gnawing on a stump of elodea.

2. The first two pairs of walking legs "hand" small bits of food to the mouth. This would be difficult if the mouth were located elsewhere. Watch a crayfish that's using those "hands" to dig in the substrate for little worms or detritus.