Define the following terms:
$(1)$ Acetabulum
$(2)$ Red muscles

(N/A) $(1)$ Acetabulum: It is a cup-like cavity formed at the point of fusion of the ilium,ischium,and pubis bones of the pelvic girdle. The head of the femur bone of the thigh articulates with this cavity to form the hip joint.
$(2)$ Red muscles: These are muscle fibers that contain a high concentration of myoglobin,which gives them a reddish appearance. They are rich in mitochondria and are capable of aerobic respiration,allowing them to store more $O_2$ and synthesize $ATP$ efficiently for sustained activity,such as the flight muscles of birds.