abdomen

abdomen in humans and other vertebrates, portion of the trunk between the diaphragm and lower pelvis. In humans the wall of the abdomen is a muscular structure covered by fascia, fat, and skin. The abdominal cavity is lined with a thin membrane, the peritoneum, which encloses the stomach, intestines, liver, and gall bladder; the pancreas, kidneys, and urinary bladder are located behind the peritoneum. The abdomen of the female also contains the ovaries, fallopian tubes, and uterus. The navel, or umbilicus, an exterior scar on the front of the abdomen, marks the point of attachment of the fetus to the maternal organism before birth. In insects, crustacea, and some other arthropods, the term abdomen refers to the entire rear portion of the body.

abdomen
1. 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 and 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.