In vertebrates, the region of the body that contains the internal organs other than the heart and lungs. In “Mammalia it is bounded anteriorly by the “diaphragm. 2. In most *arthropods, the hind region (tagma) of the body, which contains most of the digestive tract, the gonads, and the genital openings. In “Crustacea, the abdomen bears limbs which are to a greater or lesser extent segmentally arranged and the abdomen is not homologous with that of arachnids and insects. The abdomen usually shows at least some trace of segmentation, though in the course of evolution this has been lost in all but one family of spiders. 3. In insects, the segments of the body that lie posterior to the thorax. The abdominal segments carry no limbs, although there are appendages (associated with reproduction) on the terminal segments in certain exopterygote apterygote orders (e.g. *Thysanura). Non-terminal segments may bear appendages that in some insects function as gills. In the primitive state the abdomen consists of eleven segments, but this number may be very much reduced in advanced insects.