Correspindingly, echidnas are powerful differs, able to dig straight down even from a completely horizontal position. They employ a very effective defense mechansim of simply wedging themselves int a hole or crevise, spines out, and refusing to dislodge. Furthermore, echidnas can withdraw their limbs and stick out their spines, balling up protectively like a hedgehog. The second toe on each hind foot has been specially modified for grooming, cleaning and scratching. They have large, round braincases, and also demonstrate convoluted cerebral hemispheres.
Echidnas have keen olfactory abilities, but have small eyes and partically concealed ears.
It seems monotremes the excretory and reproductive ducts open to a common chamber, known as the cloaca. T. aculeatus is found over most of Australia, Tasmania, souther New Guinea and many nearby islands. It seeks shelt in burrow or crevices, and is most active in the evenings or at night. Apparently, these animals are excellent runners, supporting their body high above the ground, and can climb quite well. Their yellow and black spines grow up to 60 mm in length and are, essentially, long hollow hairs.