The ancestral lifestyle for the bivalves was one burrowed into the sediment, hidden beneath the mud or sand with only a pair of siphons protruding to draw in food particles from the surrounding environment. It was a perfectly successful lifestyle, and the majority of bivalves live in this manner to this very day. However, one clade of bivalves was determined to be different. The Pteriomorphia hauled themselves out of the mud and pursued a new lifestyle on the surface, where they diversified into such familiar bivalves as mussels, oysters and scallops. It is the scallops that I'm considering today - the pteriomorph clade that did more than merely rise to the surface. With a twitch of their shells, scallops became the only group of bivalves to fly.
The Pectinoidea include four living families - the Propeamussiidae, Entoliidae, Spondylidae and Pectinidae (Waller, 2006), as well as a few fossil families that I'm going to leave for another day. Most of you are probably familiar with the basic scallop shape - an ovoid shell, about as wide as long, round at one end, pointed at the other, with a pair of wings on either side of the point. Many species also have unequal valves to some degree, taken to an extreme in the genus Pecten which has the right valve deeply convex and the left valve entirely flat or slightly concave. Waller (2006) assigned taxa to the Pectinoidea going back to the Devonian (in the paraphyletic family Pernopectinidae), but the pectinoid crown group dates back to the early Triassic.
The main defining feature of the Pectinoidea is the unique pectinoid resilium. The resilium is the ligament that holds the two valves together at the hinge, and that of the Pectinoidea has a rubbery central core that lacks the aragonitic fibres found in other bivalve resilia. This rubbery resilium can withstand a lot more rapid compression than those of other bivalves, allowing for the unique pectinoid mode of swimming, where the rapid opening and closing of the valves pumps water to move the animal by jet propulsion. Perhaps not the most graceful forms of movement, but it does the job. Amusium pleuronectes, one of the fastest of the scallops, has been clocked travelling at 73 cm/s. And even the slower species still have the protection of their shells to fall back on if necessary. Most pectinoids start out life attached to the substrate by a byssus (beard) like mussels, but many abandon their byssus over the course of their growth and become entirely free-living. Even those forms that remain byssus-attached retain their swimming abilities. Nevertheless, a few taxa, notably the pectinid Hinnites and the Spondylidae, have abandoned the ability to swim and have become permanently cemented to the substrate.
The Entoliidae are represented in the modern fauna by the single relictual genus Pectinella, a smallish (about a centimetre across) smooth thin-shelled genus with roughly symmetrical right and left valves found in the Caribbean and tropical Pacific. The Propeamussiidae are a small-sized (often less than a centimetre), often translucent, and often strongly-ribbed family of about 200 species. The dentition at the hinge is reduced or even absent, the gill structure is simplified, and many species lack eyes or guard tenticles around the edge of the mantle.
The Pectinidae and Spondylidae both include forms much larger in size than the other families. A number of anatomical features are shared between these two families - most notably their unique eye structure*, shown above in a figure from Speiser & Johnsen (2008). Pectinid and spondylid scallops have the best-developed eyes of any bivalve, dotted around the outside of the mantle. While most bivalve eyes are little more than light detectors, serving merely to give warning to close the valves against approaching danger, pectinid eyes are developed enough to produce a rough image and can be used to direct the animal when swimming (Speiser & Johnsen, 2008). Pectinid eyes have a lens in front of not one but two retinas, one behind the other, with a concave mirror in the back of the eye. As established by Land (1965), the lens does not throw an image directly on the retina as in our eyes, rather light is reflected off the mirror at the back of the eye and an image is formed using that light by the distal retina (the retina closer to the lens). The function of the lens is to correct for distortion produced by the concave mirror. The proximal retina does not produce an image; intead, it probably functions as a light-dark receptor as in other bivalves. In the sessile Spondylus, the function of the distal retina is reduced, so it does not have the image-forming capabilities of free-swimming pectinids (Speiser & Johnsen, 2008) - without the habit of swimming, it has no need for such things. Nevertheless, the presence of the seemingly unnecessarily complex pectinid eyes in spondylids supports the idea that the sessile spondylids are derived from a free-swimming pectinid-like ancestor.
*Well, potentially unique. Pectinella does have eyes (Waller, 2006), but I haven't found a description of their structure.
Land, M. F. 1965. Image formation by a concave reflector in the eye of the scallop, Pecten maximus. J. Physiol. 179: 138-153.
Speiser, D. I., & S. Johnsen. 2008. Comparative morphology of the concave mirror eyes of scallops (Pectinoidea). Amer. Malac. Bull. 26; 27-33.
Waller, T. R. 2006. Phylogeny of families in the Pectinoidea (Mollusca: Bivalvia): importance of the fossil record. Zoological Journal of the Linnean Society 148 (3): 313-342.