Which of the following are the only vertebrates in which blood flows directly from respiratory organs to body?

Question

....tissues without first returning to the heart?

a birds

b reptiles

c mammals

d amphibians

e fishes

Answer 1

E. fish since they have a single loop circulation.  The others contain a double loop circulation allowing the blood to go back to the heart before it goes to the body.

Answer 2

The anatomy of each species is different.  And theses differences become more noticeable the further you travel back in evolutionary history.

Fish are an early vertebrate and have a relatively simple heart.  It comprises of 2 main chambers, the atrium and the ventricle.  Blood from the animals body enters the heart and is pumped forward to the gills.  Once it has been oxygenated it continues its flow around the body once more.

Other families display different circulatory systems that may be more complex e.g. mammals, or even more simple... try looking up insects!