This week's highlight taxon (sorry it's a little late, my partner's been sick) is the Rhynchonellata. Rhynchonellata are one of the major clades of the brachiopods, marine animals that look rather like bivalves (clams, mussels, etc.) on the outside, but completely different on the inside. Textbooks will usually tell you that brachiopods are commonly known as "lamp shells" (supposedly because the shell of some species bears a vague resemblance to an archaic lamp), but I can't say as I know if anyone actually uses that name. Rather, as with bryozoans supposedly being "moss animals", this seems to be a name promulgated in textbooks only. Anybody actually interested enough to discover the existence of brachiopods is usually interested enough to not be too scared by technical terminology to refer to brachiopods as anything other than "brachiopods". Living brachiopods are few and far between (Prothero, 1998, gives the figure of less than 120 living genera - the photo at the top of this post from Treasures of the Sea shows a cluster of one example, Liothyrella neozelanica) but were one of the dominant life-forms in the Palaeozoic. I must confess ignorance (but a fair amount of curiosity) as to exactly why brachiopods failed to hold onto their position of dominance. Successive mass extinctions (particularly the major die-off at the end of the Permian) seem to have decimated the brachiopods. Perhaps the main clue lies not in the failure of the brachiopods, but in the success of the bivalves - during the Palaeozoic, brachiopods excluded bivalves from all but the more marginal habitats, but bivalves sailed through the end-Permian event relatively unaffected, seemingly to claim the niches the brachiopods had left vacant before the brachiopods were able to reclaim their territory.
The diagram above (from Palaeos) shows the internal anatomy of a brachiopod, and just how different it is from a bivalve. Notably, while the two shells of a bivalve are actually on the left and right side of the animal, the shells of a brachiopod are actually dorsal and ventral. Many brachiopods are attached to the substrate by a pedicle, a fleshy stalk emerging from the back end of the shell through an opening called the delthyrium. The majority of the brachiopod's organs take up very little space in the shell, with most of the space being taken up by the lophophores, pinnate tentacle-like structures that are used to filter food particles out of the water. Because of the lophophore, brachiopods were previously regarded as forming a clade with bryozoans and phoronids (also possessing lophophore-type structures) called Lophophorata. Though the Lophophorata concept was accepted for a great many years, recent analyses have failed to support it. The lophophore in bryozoans is derived embryonically in a decidedly different manner from that of brachiopods and phoronids (Nielsen, 2002 - brachiopods and phoronids still form a clade, the Brachiozoa). Notably, the pterobranchs, a group of small sessile organisms undoubtedly belonging to the deuterostomes, actually have a lophophore-type structure much more like the brachiozoan structure than the bryozoan structure is. Molecular and possibly palaeontological* data place brachiopods somewhere close to the molluscs and annelids, but exact relationships between the three are still unclear.
*Depending on whether the suggested relationship between the brachiopods and the Cambrian halkieriids (e.g. Holmer et al., 2002) holds up.
Rhynchonellata are the major (and the only living) clade of the articulate brachiopods, distinguished from the paraphyletic "inarticulate" brachiopods by the development of proper hinge with sockets and teeth holding together the valves (inarticulate brachiopods use muscles to hold the valves together). Rhynchonellates are distinguished from the Strophomenata, the other (extinct) clade of articulate brachiopods by the absence of a pseudodeltidium (an internal shell plate partially covering the delthyrium) and the presence of a pedicle emerging posteriorly through the delthyrium (Sutton et al., 2005 - ummm, wouldn't these be plesiomorphic characters? Palaeos lists characters of the articulation supporting Rhynchonellata, but unfortunately the source sites linked to appear to be no longer available).
The majority of living rhynchonellates belong to the Terebratulida, including Liothyrella. The Palaeozoic rhynchonellates, of course, showed a much greater diversity, with some eight or so orders (one small living order, the Thecideida, didn't actually appear until during the Mesozoic). One extinct order, the Spiriferida, actually survived the end-Permian event and even made something of a go of it in the Triassic, not fizzing out until sometime in the Jurassic. Spiriferids are probably my favourite group in the brachiopods, going in for a decidedly baroque turn in presentation. As shown in the picture of Mucrospirifer above (from Wikipedia), some spiriferids developed greatly elongated hinge lines, giving them a distinct winged appearance. It is possible that these "wings" supported the brachiopod on soft sediment, allowing the spiriferid to live floating on mud.