The largest and most diversified class of the Crustacea; includes the shrimps, lobsters, crabs, sow bugs, beach hoppers, and their allies. The shell or carapace may be large, small, vestigial, or absent; the tail or abdomen is long or short; the eyes are generally set on movable stalks but may be sessile or even coalesced. Despite this diversity, the unity of the group is demonstrated by the following characteristics which all share. The maximum number of appendages is 19 pairs, even where there is an additional body segment such as is found in the Phyllocarida and embryonic Mysidacea. The trunk limbs are sharply differentiated into a thoracic series of eight pairs and an abdominal series of six pairs. The female genital duct always opens at the level of the sixth thoracic segment, whereas those of the male open at the level of the eighth. Additionally, the frontal organs of malacostracans differ significantly from nonmalacostracans. Even greater, more fundamental differences between malacostracans and nonmalacostracans have been found in their compound eyes.