The soft body of Mercenaria shows the prominent muscles (anterior and posterior adductors, pedal retractors, and siphonal retractor) that produced the scars on the inner shell surfaces. Mantle tissue lines both valves and produces new shell at the margin as the animal grows. In Mercenaria, the mantle edges are free (not fused) anteriorly to the ventral point of the siphonal retractor muscle, just posterior to the heel of the foot. The mantle edge is thickened by muscle fibers and has four folds: (1) the outer that secretes the shell and periostracum, (2-3) the middle, divided in this family into two folds (a smaller outer-middle that is sensory, and a larger inner-middle that controls water flow), and (4) the inner that directs foreign particles out of the mantle cavity. Posterior excurrent and incurrent siphons, formed by fusion of folds 3 and 4 (this is siphonal fusion Type B as defined by Yonge), are short and completely separate for their entire length. The larger incurrent siphon is ventral and the smaller excurrent siphon is dorsal. The excurrent siphon has a conical valvular membrane at its end, and the tip of each siphon is fringed with simple tentacles.
The largest part of the soft body is the visceral mass, which contains the digestive and reproductive systems of the clam and ends ventrally as the muscular foot. It is suspended in the mantle cavity by the anterior and posterior pedal retractor muscles. The foot is wedge-shaped, compressed, keeled ventrally, and extends slightly posteriorly as a “heel.” Serving as the main tool for burrowing into soft sediment, the foot is composed of muscle fibers and hemocoelic spaces that allow expansion when filled. The pedal gland lies in the midline of the foot, producing an elastic byssus in juvenile Mercenaria but becoming inactive in adults.
Mercenaria is a suspension- or filter-feeder, gathering suspended organic particles from the water using its ctenidia or gills. These plicated organs, which also serve in respiration, occupy most of the mantle cavity on either side of the visceral mass. Each side consists of two demibranchs, or double lamellae. Each outer and inner demibranch extends from between the labial palps, along the ventral edge of the pericardium and kidney, to the base of the siphons. The smaller and shorter outer demibranchs overlap most of the larger and longer inner demibranchs. Together the inner and outer demibranchs form a W-shape in cross section on each side of the visceral mass. Supra-axial extensions of the outer demibranchs also cover the pericardial space anterior of the posterior adductor muscle. Posteriorly the ctenidia connect to the siphonal septum at the base of the siphons, creating an incurrent infrabranchial chamber below the gills and an excurrent suprabranchial chamber above the gills. Structurally, the gill filaments are connected to one another by tissue junctions (eulamellibranch gills) and interlamellar septa across the space between lamellae define water tubes within the ctenidia. As water containing food particles passes through the gills, cilia on the gill surfaces move these particles to the food groove at the free edge of each demibranch and then forward to the palps. The anterior ends of the inner demibranchs are fused to the oral groove of the palps (Stasek’s palp-ctenidia association Category II). The labial palps are elongated trigonal flaps surrounding the mouth at the anterior end of the gills. Each consists of two hemipalps, the inner surfaces of which are folded into fine ciliated ridges that further sort and channel food particles collected by the gills into the clam’s mouth.
The mouth leads to a short esophagus that passes posterodorsally to the stomach. The globose stomach lies in the dorsal part of the visceral mass, surrounded anteriorly by the digestive gland where assimilation of edible food particles takes place. Internally the stomach has ciliated sorting areas, ducts to the digestive gland (here mainly concentrated into two outpockets or caeca; this defines Purchon’s Type V stomach), and an enzyme-packed crystalline style secreted by a ventral extension called the style sack. The crystalline style rotates against a chitinous gastric shield covering the interior roof of the stomach. The style grinds against the gastric shield to break up and begin digestion of edible food particles. Ciliated tracts and grooves move particles into the ducts to the digestive gland, and large inedible particles pass into the midgut (the first portion of the intestine), which is fused to the style sack in this species. The midgut undergoes a series of loops in the visceral mass, then exits the visceral mass as the hindgut. The hindgut passes through the ventricle of the heart, acting as a stabilizing structure for the contracting ventricle, then dorsally over the posterior adductor muscle to terminate as the rectum near the inner opening of the excurrent siphon. Waste products are readily passed out of the clam along with excurrent water through this siphon.
The heart surrounded by its large pericardium lies dorsally between the visceral mass and posterior adductor muscle. It consists of a single ventricle and two lateral auricles. Few blood vessels exist in bivalves, but an anterior and posterior aorta are prominent exiting the heart. The posterior aorta is interrupted in members of this family by the muscular, spongy aortic bulb posterior to the pericardium. The aortae feed into numerous hemocoelic spaces throughout the body of the clam. The hemolymph or “blood” of the clam contains several types of blood cells capable of phagocytosis. The kidneys lie along the ventral side of the pericardium, communicating with the auricles and emptying their wastes into the suprabranchial chamber. Ducts from the gonads within the visceral mass also empty gametes into the suprabranchial chamber in this area, allowing them (depending on species) either to be brooded in the chamber or to be passed out of the clam through the excurrent siphon. Mercenaria sheds its eggs or sperm into the water column, where external fertilization occurs.
The nervous system (not shown in the “transparent clam”) consists of three pairs of ganglia joined by long connectives. The fused cerebropleural ganglia lie near the anterior pedal retractor muscles. The visceral ganglia lie on the anterior face of the posterior adductor muscle near the rectum. The pedal ganglia lie ventrally in the visceral mass near the distal end of the style sack. The complex system of nerves in Mercenaria mercenaria was detailed by Jones (1979).