Why have cephalopods lost or internalized their shells?

Question#29: why does the tropical ocean have a greater temperature range than the temperate ocean?

What organ is the first to receive nutrients that have been absorbed from the digestive tract?

Abird flies by moving its wings up and down. which two systems are most directly responsible for the movement of a bird's wings? a. skeletal and muscular systems b. respiratory and muscular systems c. skeletal and digestive systems d. respiratory and digestive systems