It's hard to make head or tail of a bivalve, not least because these molluscs have long lost their cephalic portions, encasing their gills, guts and gonads in shells linked by a flexible ligament and closed by strong adductor muscles. Though surprisingly motile even in the early stages of settlement, the typical pelecypod is no adventurer in adulthood; clams, cockles and arc shells live on or in aquatic sediments, digging by means of a muscular foot, while mussels, oysters and many pectinids leash themslves to rock or rubble using sticky byssal threads.
Most bivalves maintain intercourse with the world at large via two extensible siphons, which protrude from the rear of the animal. The inhalant tube takes in fresh water for respiration, along with planktonic cells or organic deposits which are sorted in the mantle cavity, filtered by rows of ciliated lamellae and channeled to a ridged stomach. Shit and wastewater, as well as eggs and sperm, emerge from the exhalant siphon, completing a loop of vital functions regulated by a brain that is barely more than a few threads of nerve clusters. A few groups, such as solemyids and deep sea mytilids, harbour symbiotes that convert sulphides or methane into nutrients, while heart cockles and giant clams get by with a little help from single-celled friends, having borrowed a trick from zooxanthellate anthozoans.
Members of two groups of pteriomorphs, however, have learned to live with lower degrees of attachment; some scallops and file shells eschew permanent confinement, living on sandy bottoms or in crevices, from which they sieve for food and surprise predators such as echinoderms and carnivorous gastropods by vigorously flapping their valves, producing jets that propel the bivalve through the water and away from danger. Both pectinids and limids have pallial eyes lining the mantle edge, though those of the former are rather more well developed, with a cornea, two retinal layers and a lens little smaller than that of some mobile phone cameras. These optics clearly serve to detect approaching threats, but their resolving power, which has been compared to Newtonian telescopes in their ability to correct for spherical aberration, appears to have outpaced the ability of the scallop to process the pixels, much less picture the scene beyond its shells. Some correlation between visual acuity and swimming prowess has been observed, so it is possible that active clades derive an advantage from being able to somehow see where they are going or suss out suitable habitats. (Tridacnines also possess eyes, which despite being mere pinholes, are sharp enough to tell the animal where to squirt and probably help juveniles determine if they should move to a sunnier spot.)
Like scallops, limoideans have just one adductor muscle and may sport two small ears by the dorsal margin, but the shells of these unique bivalves, which bear ruts that feel rough to the touch, are usually deep rather than round. The foot, as in most non-burrowing molluscs, is short, functioning primarily to place byssal threads on hard substrates.
Limids are seldom seen during intertidal surveys, though this may be less a measure of scarcity than a result of their fondness for hidden cracks. But once spotted, file shells are unmistakable, save perhaps for some strange sea anemone should the creature behind the gape remain out of sight. For the mantle edge, which is richly pigmented and envelops the entire shell margin, is fringed by numerous long tentacles. Individual strands may be shed to ward off predators but the tentacles also play a row in locomotion, contracting en masse to help lift the animal from the seabed. Unlike scallops, Lima swims with its valves perpendicular to the substratum, sacrificing speed for staying power with the aid of lightly built shells and the voluminous pallial appendages. Elusive, disgustingly enduring and at times quite electrifying, these little clams defy the conventions of their class to float like fleshy butterflies and stick their necks out in a reef where life resists attempts to make heads or tails of its magic and majesty.