22 



MARINE AND FISHERIES 



1-2 EDWARD VII., A. 1902 



and running parallel with the lower margin of the mantle on each side is a band of 

 pallial muscles. At the posterior end of the animal are the retracted siphons, which, 

 on account of the condensation of their epidermal layer, now appear quite black. The 

 rest of the surface consists of the thin mantle, which may however permit faint outlines 

 of underlying organs to be seen. 



Mantle. — The mantle or pallium is a broad, thin lamella, hanging down on each 

 side of the animal between the body and the shell. It occupies the same position with 

 reference to the body and the shell that the fly leaves of a book do to the printed pages 

 and the backs In this species the lower margins of the two flaps of the mantle are 

 grown together, so that it is more like one's vest buttoned up the front, while the valves 

 of the shell may be compared with an unbuttoned coat. There is this difference, how- 

 ever, that the mantle and the shell are real parts of the animal, and are attached firmly 

 to the body along the dorsal line. 



The siphons are really outgrowths of the posterior margins of the mantle, that 

 have become united, developed their muscles, and have been otherwise specialized to 

 perform a definite function. There are species of clam that have no siphons and the 

 two flaps of the mantle remain separate all the way around excepting along the dorsal 

 line. Then again there are others in which the posterior margins of the mantle flaps 

 lie together in such a way as to form two openings that act as short siphons. In some 

 the siphons grow out and remain separate. In this species the margins of the two 

 mantle flaps have grown together all the way round with the exception of three small 

 areas — one the split at the anterior end through which can protrude the foot, the other 

 two being the dorsal and ventral siphonal openings. The walls around these latter 

 have become extended backwards but the part separating the two openings has remained 

 single, forming the ventral boundary of the upper tube as well as the dorsal boundary 

 of the lower. The united siphons, thus originated, have increased their length and 

 strengthened their circular and longitudinal muscles. The pallial muscles of the region 

 have become the retractor muscles of the siphons, keeping pace with the growth of the 

 latter, while their point of attachment has moved forward, occasioning the indentation 

 in the pallial line already mentioned. 



Branchial Chamber. — Make a longitudinal incision along the median ventral line of 

 the mantle, carrying it back as far as to the base of the ventral siphon and forward 

 through the anterior adductor muscle. Raise the upper, left half of the mantle and 

 there will now be exposed the large branchial chamber with its contents. Posteriorly 

 it will be seen to open to the outside through the ventral siphon, which is also called the 

 branchial siphon. The retractor muscles of the siphons show through the mantle walls. 

 The borders of the mantle are thickened and contain the glands that secrete the shell 

 substance, which is built by the deposition of new matter at the edge. These glands 

 can only be found by examining thin sections with the microscope, but at each side of 

 the foot slit, on the inside, there is a patch of mucin-glands that in colour and structure 

 are well marked from the surrounding tissue. (Fig. 4.) 



Ahdornen. — Occupying the anterior half of the mantle cavity is the plump, soft, 

 fleshy abdomen or visceral mass. It contains the stomach and greater part of the 

 intestines, the liver and genital glands. 



Foot. — Anteriorly and ventrally the walls of the abdomen become more muscular 

 and give rise to the small, extensible foot. This may contract to a mere knob, or be 

 extended to a tongue shaped or even long, thin, ribbon shaped process. The foot is the 

 locomotory organ of the clam. 



Gills. — Suspended from the dorsal wall of the branchial cavity are four long, flat, 

 striated plates — two on the left and two on the right side of the abdomen and extending 

 back to near the base of the siphons. These are the gills or branchiae. Each is com- 

 posed of two thin leaves or lamellae grown together along lines running upwards and 

 backwards in such a way as to make a large number of nearly vertical water tubes, that 

 open above into another chamber shut ofl" fronl the branchial cavity. The lamella 

 forming either surface of a single gill is perforated by gill slits arranged in rows corres- 

 ponding with the water tubes. The sides of the gill slits are clothed with fine hair-like 

 processes called cilia, that keep up such a vibratory motion as to drive water, brought 

 into the branchial cavity by the branchial siphon, through the gill slits and water tubes 



