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Abdomen 1. In vertebrates, the region of the body that contains the internal or¬gans other than the heart and lungs. In 'Mammalia it is bounded anteriorly by the "diaphragm. 2. In most 'arthropods, thehind region (tagma) of thebody, which contains most of the digestive tract, the gonads, and the genital openings. In 'Crustacea, (he abdomen bears limbs which are to a greater or lesser extent seg-mentalry 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 spi¬ders. 3. In insects, the segments of the body that lie posterior to the thorax.

The abdominal segments carry no limbs, although there are appendages (associ¬ated with reproduction) on the terminal segments in certain exopterygote and apterygote orders (e.g. Thysanura). Non¬terminal segments may bear appendagesthat 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.