Abdomen

Overview
The abdomen is the part of the body that lies between the chest and the thigh and encloses the ureters, intestines, liver, anus, bladder, gallbladder, and reproductive system outside the breast. It is also called the belly or venter. In humans, and in many other vertebrates, it is the region between the thorax and the pelvis (Separating the thoracic cavity and the abdominal cavity is the thoracic diaphragm). In fully developed insects, the abdomen is the third (or posterior) segment, after the head and thorax.

Vertebrates
In vertebrates, the abdomen contains several organs:
 * Part of the digestive system (gallbladder, liver, anus, appendix, rectum, and intestines),
 * Part of the urinary system (ureters, bladder, urethra).
 * The internal reproductive organs (ovaries, fallopian tubes, uterus, vulva, and vagina in women, testes, vas deferens, seminal vesicles, prostate, penis and urethra in men)

The abdomen also contains some of the largest and most easily accessible blood vessels in many animals, and is often used in medicine experimentation for catheterisation.

For various reasons, the abdomen is often coloured differently from the rest of the body. In animals with furry or hairy bodies, the abdomen is frequently hairless, or nearly so.

The abdomen is oval in shape and is the largest cavity in the body. It can be broken down into the lower and upper extremity. The lower extremity covers the inner surface of the bony pelvis. The Levator ani and Coccygeus are located on either side

The diaphragm forms the upper extremity and acts as a dome over the abdomen extending to the upper border of the fifth rib. It is an element in the anterior chain.

Invertebrates
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.

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.