The basic design is a segment with an optional pair of appendages. To make each functional unit, segments can be connected end to end or fused. Abdomen and thorax are sequences of segments. The abdomen, without appendages, contains the major organs, and the thorax contains muscles for legs and wings. The thorax and head of a crustacean may be fused and covered with a carapace. The internal organs are also built, generally, of repeated segments. The nervous system is built of paired cords in each segment connected to each other through the segments and crossing in each segment like steps of a ladder. Then there’s the problem of segments in the head. The head is made of fused segments that operate various appendages—pedipalps, antenna, and mouth-parts— and contains the brain, saliva gland, and main sensory organs. Different arthropods have different arrangements. Myriapods, on hatching, start with only a few segments but then they add segments and legs each time they moult. Except for the heads, the design is simple.