Abdomen

In vertebrates such as mammals the abdomen (belly) constitutes the part of the body between the thorax (chest) and pelvis. The region enclosed by the abdomen is termed the abdominal cavity. In arthropods it is the most distal section of the body which lies behind the thorax or cephalothorax.

Vertebrates

In vertebrates, the abdomen is a large cavity enclosed by the abdominal muscles, ventraly and lateraly, and by the vertebral column dorsally. Lower ribs can also enclose ventral and lateral walls. The abdominal cavity is continuous with the pelvic cavity. It is separated from the thoracic cavity by the diaphragm. Structures such as the aorta, inferior vena cava and esophagus pass through the diaphragm. Both the abdominal and pelvic cavities are lined by a serous membrane known as the parietal peritoneum. This membrane is continuous with the visceral peritoneum lining the organs. The abdomen in vertebrates contains a number of organs belonging, for instance, to the digestive tract and urinary system.

Abdominal organs

• Digestive tract: Stomach, small intestine, large intestine with cecum and appendix
• Accessory organs of the digestive tract: Liver, gallbladder and pancreas
• Urinary system: Kidneys and ureters
• Other organs: Spleen

Abdominal organs can be highly specialized in some animals. For example the stomach of ruminants (a suborder of mammals) is divided into four chambers - rumen, reticulum, omasum and abomasum.

Invertebrates

(In the worker ant, the abdomen consists of the propodeum fused to the thorax and the metasoma, itself divided into the narrow petiole and bulbous gaster.)

The invertebrate abdomen is built up of a series of concave upper plates known as tergites and convex lower plates known as sternites, the whole being held together by a tough yet stretchable membrane.

The abdomen contains the insect's digestive tract and reproductive organs, it consists of eleven segments in most orders of insects though the eleventh segment is absent in the adult of most higher orders. The number of these segments does vary from species to species with the number of segments visible reduced to only seven in the common honeybee. In the Collembola (Springtails) the abdomen has only six segments.

The abdomen is sometimes highly modified. In ants, the first segment of the abdomen is fused to the thorax and called the propodeum. The second segment forms the narrow petiole. Some ants have an additional postpetiole segment, and the remaining segments form the bulbous gaster. The petiole and gaster (abdominal segments 2 and onward) are collectively called the metasoma.

Unlike other Arthropods, insects possess no legs on the abdomen in adult form, though the Protura do have rudimentary leg-like appendages on the first three abdominal segments, and Archaeognatha possess small, articulated "styli" which are sometimes considered to be rudimentary appendages. Many larval insects including the Lepidoptera and the Symphyta (Sawflies) have fleshy appendages called prolegs on their abdominal segments (as well as their more familiar thoracic legs), which allow them to grip onto the edges of plant leaves as they walk around.