Within the genus Euglena, the subgroup "Serpentes" is characterised by species with long, slim cell bodies, which move without flagellum by snake-like locomotion in the detritus or in the mud, or swim freely in the water with a flagellum. Two major groups can be distinguished. The first is centred around the species Euglena satelles, with Euglena carterae, Euglena adhaerens and others, and is characterised by a straight-ended anterior part of the cell without a protruding flagellum. The second group is centred around the species Euglena deses, with its varieties, and Euglena ehrenbergii, and is characterised by a lateral canal opening at the anterior end with one flagellum protruding sideways. The representatives of the whole Serpentes group have various (15-30) large chloroplasts containing characteristic naked pyrenoids. The exception is Euglena ehrenbergii, which possesses innumerable small chloroplasts without pyrenoids. To better characterise this whole subgroup, to better taxonomically distinguish between the diverse species and to provide a basis for further molecular-genetic analysis of the phylogeny of and relationship between the Euglena species, we used transmission and scanning electron microscopy to investigate the five selected species. One important distinguishing feature among the species is the form of the pellicle. It can differ in thickness or cross-sectional shape (e.g. A-, M-or plateau-like shape) and can have various arrangements of microtubules and endoplasmic reticulum mucus vesicles. We show that the group is more heterogeneous than expected and that some species have very individual features that poorly fit into a common Serpentes group, particularly the above-mentioned Euglena ehrenbergii. Euglena carterae, formerly named Euglena deses var. carterae, with its typical straight-ended canal opening, does not fit into the Euglena deses varieties, as has already been confirmed by molecular genetic methods.

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