folds, which enclose a large space, the mantle or branchial chamber. This 

 is essentially a closed space, excepting for the siphonal openings. Two 

 large, conspicuous folds, the gills, arising from the side of the body, hang 

 free in the mantle chamber. They perform an important function in addition 

 to aeration of the blood - that of food collection. Just anterior to the 

 gills, and behind the large anterior adductor muscle, are two small folds, 

 the labial palps. The mouth opening is on the median line behind the ante- 

 rior adductor muscle, hidden from view by the closely applied palps. It is 

 a funnellike entrance to the digestive tract. When gills are removed, the 

 main mass of the body is exposed, made up chiefly of a large colored gland, 

 the digestive gland, and the greatly developed sexual organs. This body is 

 called the visceral mass, to distinguish it from the muscular foot on its 

 ventral surface. In a large clam the foot may be projected two or three 

 inches from the edge of the shell. Its distension is caused by a large 

 quantity of blood pumped from the heart. The foot is an organ of locomotion, 

 and is also used in burrowing. The gill is not a solid flap or fold, but a 

 minute basketlike structure with an outer and inner wall enclosing a space 

 between. The walls are made of extremely fine rods placed side by side, and 

 irregularly fused with each other by secondary lateral growths of tissue. 

 Outer and inner walls of the gill are also held together by partitions 

 between them. The gill is thus basketlike, the walls made up of rods between 

 which are spaces, which put the interior chamber in communication with the 

 mantle space. The rods contain an interior space in which blood flows. 

 Microscopic study shows that the rods are covered on their outer surfaces 

 with innumerable short cilia. Other cilia are present on the rods than 

 those which cause water to enter the gills. These long cilia are arranged 

 in bands which project out laterally between contiguous filaments so as to 

 strain the water which enters the gill, thus preventing all floating matter 

 from entering. The food of the clam is almost instantly cemented together 

 by a sticky mucus, secreted by special glands in the rods or filaments, and 

 the whole mass moves with some velocity toward the free edge of the gill. 

 On this free margin is a groove into which material collected on tne faces 

 of the gill is turned. This groove is also lined with ciliated cells, and 

 the whole mass is carried forward toward the palps. The palps are covered 

 on their inner surfaces nearest the mouth with a set of very fine parallel 

 ridges, which are capable of many movements. When their inner faces touch 

 the edges of the gills any material being brought to this region is trans- 

 ferred onto tne ridges of the palp. Cilia on the palp carry the matter on 

 across the ridges and finally force it into the mouth. Experiments with 

 grains of carmine show that definite cilia currents exist. All converge at 

 a definite point just above the line of the base of the muscular foot. 

 Material which touches this surface, instead of being taken toward the mouth, 

 is forced in the opposite direction. Clams undisturbed in the bottom may be 

 seen from time to time to discharge a strong jet of water from both siphons. 

 This serves the purpose of removing masses of material which the animal can 

 not use as food. This is not the only means of discharging undesirable 

 material. The mantle is also ciliated. Everything is swept downward toward 

 the free edge of the mantle, falls into a line parallel with the edge, and 

 is then directed backward, but can not be carried out of the incurrent siphon 

 against the stream which is entering. In a little bay beneath the base of 

 the siphon, where it is out of the current, the material is collected. By 

 contraction of the adductor muscles, and the resulting emptying of the mantle 

 chamber, the collected mass is expelled. If much mud is entering large 

 quantities of it must be collected on the gills and be sent forward toward 

 the mouth. If the palps are withdrawn so as not to touch the gills, material 

 will accumulate in the anterior parts of the gill grooves until the masses 

 are so large that they fall off into the space of the mantle chamber below. 

 They would be discharged when the mantle space was emptied. Close examination 

 of the inner faces of the palp shows a narrow strip around its margin which 

 is without ridges. Both of these margins are very densely ciliated. When 

 suspended material falls on the upper margin, it is carried up onto the sur- 

 face of the ridges and across them to the mouth. Anything which touches the 

 other margin is swept with great rapidity in the other direction, out to the 

 end of the palp, where it accumulates and is finally thrown off into the 

 mantle chamber below. Enough has been said about the food of Venus to make 

 it clear that if it were raised on beaches or flats we should not find as 

 rapid a growth as if it were never exposed, for feeding is impossible with- 

 out water currents. In one experiment in the natural environment several 



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