Biogeography is the study of the distribution of species and ecosystems in geographic space and through geological time. Euglena possesses chlorophyll similar to plants and thus synthesizes food in presence of light by photosynthesis. It lacks cell wall and also acts as heterotroph in absence of light that are the characters of animals. Thus, it is said to be a connecting link between plants and animals. The composition and physiological role of blood, lymph and tissue fluid is almost similar in all vertebrates.

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