Crocodilia - Description - Internal Organs

Internal Organs

Crocodilians lack a vomeronasal organ (except in the embryonic stage) and a urinary bladder.

Like mammals and birds and unlike other reptiles, crocodilians have a four-chambered heart. While the four-chambered heart is traditionally characteristic of endotherms, it is thought that the ectothermic crocodilia have a four-chambered heart because of an endothermic ancestry, originating in the archosaurs or in an earlier predecessor. When crocodilian ancestors transitioned back to aquatic ectothermy, it was advantageous for them to have a heart more akin to the normally three- or five-chambered heart found in most ectotherms. In order for their four-chambered heart to function more like the ectothermic heart, they adopted a mechanism for shunting blood in an alternative pathway through the heart. The right ventricle has two arteries leaving it; a pulmonary artery, which goes to the lungs, and the left aortic arch, which goes to the body, or systemic circulation. There is also a hole, the foramen of Panizza, between the left and right aortic arches. Because the left aortic arch goes directly to the gut, the shunting of oxygen depleted blood which is high in CO₂ may serve to aid in creating stomach acid to assist in digesting bones from its prey. Their blood has been shown to have strong antibacterial properties.

Crocodilians have lungs with alveoli. They have a unique muscle called the diaphragmaticus that attaches to the liver and viscera and acts as a piston to assist in breathing. The diaphragmaticus is not homologous to the diaphragm of mammals and the proto-diaphragm of tegu lizards. Like other amniotes, crocodilian breathing uses muscles between the ribs to both increase and decrease thoracic volume. In addition, expiration is accomplished by contracting muscles to move the liver towards the head to rotate the pubic bones to decrease abdominal volume. Inspiration involves contraction of the diaphragmaticus muscle to push organs to the back of the body and other muscles to make space for these organs. In crocodilians, expiration is mostly passive (involves little muscle contraction) during rest while inspiration always involves muscle contraction. Because many of these ventilatory muscles are used for maneuverability in water, and because the muscles were originally used for locomotion, it is possible that these muscles became ventilatory muscles after they evolved to move air around in the lungs for maneuverability.

Crocodilians are known to swallow stones, gastroliths ("stomach-stones"), which act as a ballast in addition to aiding post-digestion processing of their prey. The crocodilian stomach is divided into two chambers; the first one is described as being powerful and muscular, like a bird gizzard. This is where the gastroliths are found. The other stomach has the most acidic digestive system of any animal, and it can digest mostly everything from their prey; bones, feathers, and horns.

Crocodilian sex determination is temperature-dependent; i.e., the sex of developing crocodilians is determined by the incubation temperature of the eggs. This means crocodilians do not have genetic sex determination, but instead have a form of environmental sex determination which is based upon the temperature embryos are subjected to early in their development.