Euglena, introductory notes
For illustrations to accompany these notes see Protista
The many species of Euglena and some of the related organisms, all called flagellates, form an interesting group of unicellular creatures since, although the majority are plant-like in their feeding, a few feed saprophytically (like fungi), some holozoically (like animals), and nearly all of them possess characteristics which make them more like animals than plants.
Unlike chlamydomonas, and green plants in general, euglena does not have a rigid cell wall and can change its shape as it swims along, though movement is effected by the lashing of the flagellum.
Food is synthesized, as in green plants, with the aid of chloroplasts the shapes of which vary in the different species. If Euglena gracilis is kept in the dark it will lose its green colour and become unable to photosynthesize, but it will continue to live if suitable organic matter is present in the water. In this respect it is very similar (some biologists think it is identical) to some of its relatives in the genus Astasia. These colourless flagellates live only in water rich in organic materials such as the fluid in the puddles round cowsheds. They absorb the organic substances from their environment and use them as food, i.e. they feed saprophytically, although they may still be able to synthesize complex materials from relatively simple inorganic chemicals. They cannot, however, build up starch from carbon dioxide. Other colourless flagellates are able to take in solid particles of food material and digest them in a way described in the section dealing with Paramecium. This holozoic feeding is essentially an animal characteristic. The distinction between holozoic and holophytic feeding is the fundamental difference between animals and plants.
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