Question

Blood of earthworm is red because its haemoglobin is?

Answer 1

Haemoglobin is the iron-containing oxygen-transport metalloprotein found in the red blood cells (erythrocytes) of almost all vertebrates (the exception being the fish family Channichthyidae) as well as the tissues of some invertebrates. The worm's hemoglobin has evolved to transport oxygen for the worm.

Complete Answer: Earthworm blood – unlike that of most invertebrates – contains haemoglobin, a protein which carries oxygen throughout its body. It is present in the blood plasma and contains a red-coloured compound called heme. Heme contains an iron atom that binds to oxygen. The oxygen is picked up by the haemoglobin dissolved in blood plasma and carbon dioxide is released.
Earthworms build enormous haemoglobin complexes to carry their oxygen. These complexes are termed as erythrocruorin. It is composed of 144 separate globin chains, each with its own heme group to carry oxygen. These globin chains come in four similar types (36 copies of each) similar to the slightly different alpha and beta chains found in human haemoglobin. Then, 36 linker protein chains stitch all the globin chains together into one big complex. In earthworms, gases are exchanged through the moist skin and capillaries.

Note:
Earthworms are one of the few organisms that have haemoglobin without any red blood cells. Its affinity for oxygen is weaker than that of most haemoglobins. The enormous macromolecule, erythrocruorin, is typically found free floating in the plasma.