Complex sentence: Euglenids are distinguished mainly by the presence of a pellicle (periplast), which is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventral microtubules.
Split into simpler sentences: Euglenids are distinguished mainly by the presence of a pellicle (periplast). The pellicle is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventral microtubules.