CONCLUSION
Now that you have learned this technique, you can apply it to analyze the blood of other animals. For example, observe the blood of earthworms, which are easy to find. What you see even in the blood of simple animals is very interesting. As the blood of vertebrates is the fruit of a long evolution, as you descend the evolutionary ladder, you will see simpler types of blood, but you will also find a continuity which will help you to understand the blood of different animals.
In the more primitive beings, the liquid which flows among the cells has a composition very close to that of the water and performs modest functions. If you go up the evolutionary tree, this liquid assumes new and more complex functions. While in the invertebrates, the blood, called hemolymph, wet the organs and only in a part flows inside vessels, in the vertebrates blood flows in a vascular system which is entirely contained by walls and the cells are wetted by lymph instead. In the vertebrates, the blood also carries out complex functions of transport, homeostasis and defense.
As for the immune system, even in the protists there is a very rudimentary form of recognition of what is extraneous. Of course, in this case it is too far removed to be called an immune system! However, in the comparatively simple multicellular organisms, such as annelids and arthropods, cells do exist with defensive functions but they do not perform a specific action. Often, they are generically named phagocytes, other times with more specific terms. In the vertebrates, lymphocytes occur, defensive cells endowed with specific roles.
We hope this experiment has helped you to become familiar with the blood and the methods for the microscopic observation of its cells. This may be useful as a science experiment during school, or even as general knowledge. To an amateur microscopist it may be a stimulating experiment. However, what you will observe during these experiments will help you to follow the development of life in our planet, even if from an unusual point of view.