GENERAL ZOOLOGY 



The major portion of the body, the visceral mass, occupies the central part 

 of the mantle cavity. The foot is an extensive, flexible, muscular region 

 attached along the ventral margin of the visceral mass and continuous with it. 

 On each side of the body a pair of plate-like gills, each composed of rows of 

 parallel water tubes, hangs down into the mantle cavity between the visceral 

 mass and the mantle (Figs. 13.1, 13.2). Anterior to the gills are the labial 

 palps, which, like the gills, are paired. The mouth lies in the angle between 

 the anterior end of the foot and the anterior adductor muscle, and the palps 

 are connected across the midline by ridges which extend transversely and form 

 lip-like structures above and below the mouth. 



In its normal position, embedded in the sandy or muddy bottom of a body 

 of water, the animal usually lies with the median plane vertical and only the 

 posterodorsal margins of the valves visible. Ordinarily, the valves are held 

 slightly agape, and the siphons protrude into the water. The action of cilia 

 on surfaces bounding the mantle cavity maintains a gentle circulation of 

 water, passing in through the ventral siphon, circulating in the mantle cavity 

 and through the gills, and finally moving outward through the dorsal siphon. 

 In locomotion the foot is thrust forward and either expanded at its tip or 

 turned so that it takes a clumsy hold in the sand or mud of the bottom. The 

 body is then drawn forward by the contraction of the powerful muscles of 

 the foot. 



Structures and Functions Related to Metabolism and Responsiveness. 

 During the following discussion, it should be borne in mind that the entire 

 "economy" of the clam is based on the circulation of water through the 

 mantle cavity. Powerful cilia on the mantle, on the surfaces of the visceral 

 mass, on the gills, and on the palps function to maintain this circulation. 

 The organism depends on these currents of water for food, for the elimination 

 of egesta and excreta, for exchange of respiratory gases, and very often for 

 the dispersal of gametes and zygotes. 



The water entering the mantle cavity through the incurrent siphon is drawn 

 through the ostia, minute openings on the surfaces of the gills, into the water 

 tubes, and through these vertical passages upward to the suprabranchial 

 chambers. There are four of these chambers, one lying above each gill and 

 parallel to the longitudinal axis of the body. The suprabranchial chambers 

 unite beneath the posterior adductor muscle to form a region known as the 

 excurrent chamber, or cloaca, which opens externally by way of the excurrent 

 siphon. In the course of this circulation through the gills, gaseous exchange 

 is effected between the water and the circulating blood. Microscopic particles 

 of food borne by the water entering the mantle cavity are sifted out by the 

 cilia on the gills, or are entangled in sheets of mucus secreted by the mantle 

 and the surface of the visceral mass. These food particles and food-laden 

 sheets of mucus are moved toward the palps by specifically oriented ciliary 

 pathways, and finally are conducted into the mouth by cilia on the palps. 

 The palps appear to exercise a certain amou-nt of selectivity, largely on the 

 basis of weight: heavier particles are dropped by the palps and gather 



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