A low hemoglobin count can also be due to blood loss, which can occur because of: 1  Bleeding from a wound. 2  Bleeding in your digestive tract, such as from ulcers, cancers or hemorrhoids. 3  Bleeding in your urinary tract.  Frequent blood 1  donation. Heavy menstrual bleeding.
The brain is the control centre for your body and it sits in your skull at the top of your spinal cord. The brain has three main parts. 1  The cerebellum (say se-re-bell-um). The brain stem, that controls a lot of the 'automatic' actions of your body such as breathing and heart beat, and links the brain to the spinal cord and the rest of the body.
The endoskeleton and muscles form an organ system that permits rapid and efficient movement. The pectoral and pelvic fins of fishes evolved into jointed appendages that allowed vertebrates to move onto land. The skull, the most anterior component of the main axis of the vertebrate endoskeleton, encases the brain.embers of the class Myxini have a partial cranium (skull), but no vertebrae. Their skeleton is made of of cartilage, as is that of sharks. Hagfish lack jaws, and for this reason used to be classified with the lampreys in a group called the Agnatha (no jaws) or the Cyclostomata (round mouth).