Following on from the post just put up on xenophyophores, I couldn't pass on without mentioning these fellows:
This is Haeckel's (1889) plate of members of the Ammoclathrinidae, taken from here. Haeckel described members of this family* among his "Deep-Sea Keratosa", the other examples of which have since been identified as xenophyophores (and not, as Haeckel thought they were, sponges). Like xenophyophores, ammoclathrinids had coverings constructed from foreign objects, and when I wrote the page on xenophyophores for Palaeos.com, I interpreted ammoclathrinids as being xenophyophores. True, Haeckel had originally identified separate cells inside his ammoclathrinids, but I suggested that this could have been an artefact caused by his use of acid to prepare the specimens, and they might have originally been coenocytic. However, when I wrote that I was working from Tendal's (1972) recounting of Haeckel's work, and hadn't seen the original description. Looking at it now, I have to admit that the illustrations in Haeckel's plate look very different from xenophyophores.
*Which he called Ammoconidae, but this name was changed by Tendal (1972) for reasons of preoccupation.
Unfortunately, Haeckel's original material no longer exists (it seems likely that he pulled it apart in the process of describing it), and no-one has ever laid eyes on a recognisable ammoclathrinid since. The nature of these organisms, therefore, is a complete mystery. Such a situation is more common than you may think, particularly in microbiology. The protistological literature is littered with examples of organisms seen and described by someone looking down a microscope in the 1800s, and never recorded since. It might be tempting to wonder whether the original describer ever really saw what they thought they saw at all, but that may be ignoring one of the major issues in microbiology itself - the world is very, very large, and the things being looked for a very, very small. If comparatively large mammal species can elude attention for numbers of years, how much more so can tiny micro-organisms? Or something like Ammoclathrinidae, lurking somewhere in the little-explored deep sea?
Haeckel E. 1889. Report on the Deep-Sea Keratosa. Report on the Scientific Results of the Voyage of H. M. S. Challenger during the years 1873–76. Zoology 32 (part 82): 1–92.
Tendal, O. S. 1972. A monograph of the Xenophyophoria (Rhizopodea, Protozoa). Galathea Report 12: 7-99.