 | echidna homepage |
One of the most striking features of the monotremes is that they lay eggs from which the young are subsequently hatched. These eggs are like those of reptiles in that they have large yolks and rubbery shells. Other interesting reptilian features are seen in the structure of the urogenital organs and in the skeleton. In both sexes of the monotremes, as in all reptiles, the intestine, bladder, and reproductive organs all open into a common chamber, the cloaca, with only one external opening. The penis of monotremes is used solely for delivering sperm and not also for urination as in most other mammals. In the skeleton, the shoulder gridle not only retains well-developed coracoid bones, which are reduced to vestiges in other mammals, but also an interclavicle. The pelvic girdle carries epipubic bones, which, as in some reptiles and marsupials, act as supporting structures for the ventral (abdominal) body wall.
The skull of monotremes has a smooth, rounded cranial portion, terminating in a long rostrum, or snout. Both monotreme types are toothless as adults, and the echidnas are toothless throughout life. The platypus has teeth when young, but they are shed before the animal becomes adult and are replaced by horny pads. The testes are abdominal and there are no teats and no vibrissae (whiskers). The limbs are modified for digging or swimming. The posture of the platypus resembles that of reptiles, especially lizards. In the echidnas, however, the limbs support the body well off the ground, even when the animals are stationary or walking slowly.
An interesting feature of the digestive system of monotremes is that the whole inner surface of the stomach is lined with a cornified (horny) epithelium. There are no glands of any kind. Particularly interesting is the lack of those that produce hydrochloric acid and peptic enzymes, which together initiate the digestion of protein in other mammals. Some other insectivorous mammals also have cornified epithelium covering most of the stomach lining, but there is a small glandular area. It has been suggested that the breakdown of food in the stomach of Tachyglossus is assisted by the grinding action of ingested dirt.
| echidna homepage |