Final answer:
The Excavata group, specifically diplomonads and parabasalids, often lack mitochondria and have flagella or deep oral grooves. These protozoans, like Giardia lamblia and Trichomonas vaginalis, possess mitosomes and hydrogenosomes respectively, which are modified forms of mitochondria.
Step-by-step explanation:
The group that often lacks mitochondria and possesses flagella and/or deep oral grooves is part of the supergroup known as Excavata. Within this supergroup, organisms such as the diplomonads and parabasalids fit this description. Diplomonads, like Giardia lamblia, lack conventional mitochondria and have organelles called mitosomes, which are thought to be remnants of mitochondria. These organisms generate energy through alternative pathways like glycolysis and have flagella, allowing them to move. Parabasalids, including the Trichomonas vaginalis which causes trichomoniasis in humans, have modified mitochondria called hydrogenosomes. It is important to note that although initially thought to be descendants of ancient eukaryotic cells completely lacking mitochondria, these species appear more likely to have once had, but then lost functional mitochondria. The present mitosomes and hydrogenosomes suggest a reductive evolution from ancestral mitochondria, adapting to life in anaerobic environments where these organisms thrive.