They have a cloaca into which the urinary and genital passages open, but not a swim bladder.
Muscle is formed of contractile filaments and is separated into three main types; smooth muscle, skeletal muscle and cardiac muscle.
Smooth muscle has no striations when examined microscopically.
The bones of mammals are well ossified and their teeth, which are usually differentiated, are coated in a layer of prismatic enamel.
The teeth are shed once (milk teeth) during the animal's lifetime or not at all, as is the case in cetaceans.
It contracts slowly but maintains contractibility over a wide range of stretch lengths.
It is found in such organs as sea anemone tentacles and the body wall of sea cucumbers.
They mostly have four limbs but some aquatic mammals have no limbs or limbs modified into fins and the forelimbs of bats are modified into wings.
The legs of most mammals are situated below the trunk, which is held well clear of the ground.
Anatomy is the branch of biology concerned with the study of the structure of organisms and their parts.
Anatomy is inherently tied to embryology, comparative anatomy, evolutionary biology, and phylogeny, as these are the processes by which anatomy is generated over immediate (embryology) and long (evolution) timescales.
Mammals have three bones in the middle ear and a cochlea in the inner ear.