Why isn't a horse crab a mollusk?

horseshoe crabs are in a class by themselves; they are more closely related to spiders, scorpions, and ticks. They are different from true crabs because they have no antennae and no mandibles (mouth parts for grinding food).

Explanation:

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.