Lampreys Completely Lack Parathyroid Glands

Lampreys completely lack parathyroid glands. Suggest why these glands may be important for other vertebrates, but not for jawless fish.

A) Because they are parasitic, lampreys get plenty of calcium from their hosts.
B) Living in the ocean, they are surrounded by large amounts of sodium, chloride, and magnesium ions.
C) Jawless fish employ a different system than other vertebrates to regulate blood sugar.
D) Since they have a cartilaginous skeleton but not true bone, they have less need to regulate calcium levels.