we look at lancelet (Fig. 1) and clam (Fig. 2) similarities.
At first clams seem odd and inscrutable,
but when you simplify their structures (Fig. 2), many previously overlooked similarities to lancelets begin to appear. Lancelets (Fig. 1) have a straight intestine terminating below a terminal tail. Clams also have a terminal tail, but it is traditionally called a foot. In clams the stomach and intestine arch dorsally and terminate dorsal to the ciliated mouth (as in lophophorates), expanding to produce a funnel, as in Nautilus (which has a ventral funnel, likely due to a close, but separate ancestry from the clam).
The phylogenetic origin of the bilateral clam shell
remains a mystery at present. Ontogeny (Fig. 3) provides clues. Clam shells develop during the clam’s planktonic (= free-swimming) embryo stage, shortly after feeding commences and prior to settling on or burrowing tail first into the sea floor (like a lancelet, Figs. 1, 2).
All molluscs are traditionally considered protostomates,
but note a subtle difference: the traditional protostomate trochophore (= early embryo) has a mouth that appears at the so-called equator (Fig. 3). By contrast the clam mouth appears in the middle of buccal cilia, at the oral pole, as in the lancelet. In clams, as in the nautilus, octopus and starfish, the buccal cilia double as organs of locomotion. This is distinct from lancelets that depend on their tail, not their mouth, to swim and dig. By this evidence, this early stage (Fig. 3) is where the switch from one to another took place, if ontogeny recapitulates phylogeny.
Both clams and lancelets have an atrium
for filtering plankton from sea water. Both have a stomach opening posterior to the atrium to collect plankton captured on mucous strands traveling along the atrial walls.
The benthic, burrowing, plankton-feeding lifestyle of a clam
(Fig. 2) remains very much like that of its unarmored ancestor, the lancelet (Fig. 1). The armored body looks extremely different, from the outside, but take away the armor and the similarities become more noticeable.
Lancelets came first.
They are closer in morphology to their elongate nematode ancestors (Fig. 4) and only develop buccal cilia as they near adulthood. In clams the buccal cilia appear early in embryology and take over as swimming organs. Timing is everything. And it looks more and more like the traditional phylum Mollusca is polyphyletic, like traditional diapsids, protorosaurs, pterodactyloids, turtles and whales.
This, too, appears to be a novel hypothesis of interrelationships.
If not please provide an earlier citation so I can promote it here.
Mansfield JH, Halaler E, Holland ND and Brent AE 2015. Development of somites and their derivatives in amphioxus, and implications for the evolution of vertebrate somites. EvoDevo 6(21): DOI 10.1186/s13227-015-0007-5
Clam hatchery: http://www.fao.org/3/y5720e/y5720e0a.htm