Two differences between the heart of a fetus and an adult heart.

As in an adult heart, the prenatal heart develops four chambers and four valves. But because the fetal lungs will not be used until after birth, blood must bypass the lungs. Two structures develop in the prenatal heart that allow the blood to be routed around the lungs: the foramen ovale and the ductus arteriosus.

the trilobite is used as an index fossil for various geologic time periods as specific types of the trilobites were present in different geological time periods. they are a group of marine arthropods belonging to the class of trilobita.

explanation:

answer: - b cells

explanation: -

so, the b cells mediate their action by identifying the antigens present on the surface of the pathogens and then producing antibodies in return to mediate its effect.