Final answer:
The correct answer is a. Basal exopod. The basal exopod and medial endopod are parts of crustacean limb anatomy, with the basal exopod being the outer branch closest to the body. The metacarpophalangeal joint is an example of a biaxial diarthrosis, which allows movement in two planes.
Step-by-step explanation:
The two joints of the protopod in crustacean limb anatomy are the basal exopod and medial endopod. Specifically, the correct answer is a. Basal exopod. The protopod functions as the base segment of the limb that supports two branches: the exopod (which is the outer branch) and the endopod (which is the inner branch). The basal part refers to the segment closest to the body. When we talk about joint classification and movement, a biaxial diarthrosis such as the metacarpophalangeal joint allows for movement in two planes.
The metacarpophalangeal joint is an articulation between the distal end of a metacarpal bone of the hand and a proximal phalanx bone of the thumb or a finger. This kind of joint is capable of movements such as flexion and extension, as well as abduction and adduction, making it a biaxial joint. In contrast, the hip joint, which is a multiaxial joint, allows for freedom of movement in multiple planes.