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Slugs and clams are both phyla in mollusca, but how can I tell which features they inherited from an ancestor? Basically, I'm curious about how excatly these organisms are related but can't find any information and do not know where to look.

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The Tree of Life web Mollusc page shows that gastropods (snails/slugs/etc.) and bivalves (clams/mussels) are two of the major groups within Mollusca:

enter image description here

You can click through to get more detail on the individual groups (but there isn't much more information about the defining characters, date of divergence, etc.). At the very least, these technical names for the taxa (and of the Conchifera, their parent taxon) should help you search for more information, e.g. a Google scholar search on "bivalvia gastropoda divergence", which gets to Investigation of molluscan phylogeny using large-subunit and small-subunit nuclear rRNA sequences ("scaphopods are more closely related to gastropods and cephalopods than to bivalves") or Phylogenetic relationships of annelids, molluscs, and arthropods evidenced from molecules and morphology. (You do have to be careful in reading the phylogenetic literature, as it's rapidly evolving; the latter paper is from 1996 and may well be superseded.)

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There isn't that much easy information for snails and clams, They diverged 550 million years ago, and their anatomies are well studied. If it wasn't for genetic evidence perhaps not all scientists would agree that they are monophyletic.

The shell is the most obvious common feature, made of silicon (nacre/pearl) and calcium, and the next nearest animal to have a similar growing shell is which animal? a wormy thing with a shell doesnt exist AFAIK. Even the shell is different:

nacre layers in gastropods and cephalods (Nautilus) are formed by the progressive nucleation of crystals at the top of conical stacks of aragonite platelets and the lateral thickening and eventual coalescing of older crystals at the base (Mutvei 1970. Wise 1970a. Wise 1970b, Erben 1972, Mutvei 1978, Nakahara et al. 1982, Wilbur and Saleuddin 1983). In contrast, in bivalves. growing sheet nacre has a steplike structure, and at each step, there is a gradient of maturing crystals overlying fully formed layers (Wise 1970a. Wise 1970b).

enter image description here Snails and Clams have nearly no articulations, only one major muscle as a result, the grinding and filter apparatus in both is very different, so that only leaves simple things like the heart and circulations and shell and muscle anatomy in common.

https://en.wikipedia.org/wiki/Evolution_of_molluscs https://www.researchgate.net/publication/259355769_The_Continuing_Debate_on_Deep_Molluscan_Phylogeny_Evidence_for_Serialia_Mollusca_Monoplacophora_Polyplacophora

perhaps someone can tell you from this image of genetic distance the percentage of the genome they have in common. https://www.researchgate.net/profile/Julia_Sigwart/publication/259355769/figure/fig2/AS:213874457485316@1428003052255/Preferred-molluscan-tree-Maximum-Likelihood-analysis-RAxML-28-hardway-of-pruned.png

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