Recent studies (e.g., Bieler et al., 2014) have confirmed that the Bivalvia consist of two major clades, Protobranchia and Autobranchia, with the latter dividing into Pteriomorphia and Heteroconchia. Heteroconchia in turn consist of Palaeoheterodonta and Heterodonta, with the latter dividing into Archiheterodonta and Euheterodonta. To avoid introducing additional Linnean ranks into the classification based on this hypothesized branching pattern of the bivalve tree, Autobranchia and Heteroconchia are not translated into ranks herein.
Bieler, R.; Mikkelsen, P. M.; Collins, T. M.; Glover, E. A.; González, V. L.; Graf, D. L.; Harper, E. M.; Healy, J.; Kawauchi, G. Y.; Sharma, P. P.; Staubach, S.; Strong, E. E.; Taylor, J. D.; Tëmkin, I.; Zardus, J. D.; Clark, S.; Guzmán, A.; McIntyre, E.; Sharp, P.; Giribet, G. (2014). Investigating the Bivalve Tree of Life – an exemplar-based approach combining molecular and novel morphological characters. Invertebrate Systematics. 28(1): 32-115.
"Cylinders may also act as pipelines carrying one material through another, like underground pipes. Man's oil or drainage pipelines are usually rigid, but in nature flexibility is more valuable for this purpose. Some bivalve molluscs, such as clams, can live quite deep under the sandy sea bed by virtue of their extensible cylindrical siphons, one for inhaling water and the other for exhaling it after the gills have extracted food and oxygen." (Foy and Oxford Scientific Films 1982:23) Learn more about this functional adaptation.
Foy, Sally; Oxford Scientific Films. 1982. The Grand Design: Form and Colour in Animals. Lingfield, Surrey, U.K.: BLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, London. 238 p.
"Another protein rubber is abductin found in the shell-opening ligaments of bivalve mollusks. One or two adductor muscles hold the two halfshells or valves of a bivalve closed (the edible part of a scallop is one of these muscles). Closing compresses the ligament, so its elastic resiliency can reopen the shell if the muscles relax. Interestingly, scallops, which swim by repeatedly clapping their valves together, recover a greater fraction of the work done on their abductin than do clams and other more sedentary forms." (Vogel 2003:304) Learn more about this functional adaptation.
Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
Barcode of Life Data Systems (BOLD) Stats Specimen Records:27911 Specimens with Sequences:22620 Specimens with Barcodes:19609 Species:1885 Species With Barcodes:1532 Public Records:16881 Public Species:1288 Public BINs:1427