| Giardia lamblia is very unusual in evolutionary terms. It seems to be an intermediate between bacteria and multicellular life. Specifically, it is a eukaryote, but does not have any mitochondria. Modern classification would make it a separate kingdom within the Eucarya, parallel to the common ancestor of all the other kingdoms.
Until very recently it was also thought to have neither endoplasmic reticulum nor Golgi apparatus. These have now been detected. This discovery confirms a theory that the cell nucleus evolved in the same evolutionary event as the endomembranes. All creatures that have one have the other. The Golgi apparatus is present throughout the life cycle but in functionally different forms, which had earlier prevented its identification.
It and some other protists lack mitochondria. This would normally be taken to mean that they are from a line of early eukaryotes that diverged before our ancestors fused with symbiotic bacteria (perhaps 1000 million years ago), as suggested in the endosymbiotic hypothesis. But Bohdan Soltys and Rad Gupta, the scientists who found the endoplasmic reticulum in 1996, also report traces of a mitochondrial protein called hsp60, which may mean that precursors to Giardia had them and lost them.
This protozoon has a trophozoite phase, that is one that adheres to the columnar cells of the villi of the intestine. It has 4 pairs of flagella and a sucking disk to attach to the surface of the villus and a beak for eating honey. This form has two nuclei. In the cyst phase found in the faeces it is a roundish thing with four nuclei. Presumably the cyst phase it what it turns into to reproduce, leaving one host and spreading into the water for others to pick up.
The diarrhoeic disease it causes is called giardiasis, or colloquially beaver fever, because it is often caught in waters where such animals have been excreting.
Giardia were among the "animalcules" observed by van Leeuwenhoek in the 1600s.
It is named for the Belgian marine biologist Alfred Giard (1846-1908), who was a Neo-Lamarckian and a professor at the Sorbonne, and founder of a laboratory at Wimereux. (He is called French by some less reliable sources I've seen; but I haven't been able to find anything much about him.) | 
