34 MOLLUSC A. 



side. Fig. 20 shows a section of the arm near the end, 

 with the gills reduced to a fleshy ridge on either side. 

 The dotted lines (^+) show how these gills can be 

 drawn to resemble those of Pecten. Fig. 18 shows an 

 actual section through the free part of the gill-arms and 

 gill-tubes in Pecten. The dotted lines {jn'^^ here also 

 indicate the mantle on both sides, and the lines (^+) 

 the way in which the gills of Pecten may be trans- 

 formed into a resemblance of those in the clam and 

 oyster. 



The blood-vessels in the fleshy bases (Fig. 18, br) are 

 very simple, one above the other; the uppermost (/) 

 conveying blood to the gills, and the lower (/-') receiv- 

 ing it from the gills. On the right, the vessels leading 

 into the gills, and their dilations into partitions, are 

 shown ; and on the left the tubes and the vessel into 

 which they carry the returning blood, and which take 

 it to the hearts. The plural is used here, for there are 

 really two of these, consisting of an auricle and ventricle, 

 one either side of the body. The gills are also very 

 interesting, because the cross tubes which unite the 

 vertical tubes internally, as seen in the figure, and the 

 outer membrane of the folds through which the still 

 smaller vessels pass between the apertures, are so very 

 thin and easily ruptured that the vertical tubes separate 

 at a touch, and in many alcohoHc specimens assume 

 the aspect of distinct tubes. Fig. 17 shows the gills 

 of one side split into five pieces. 



The so-called organs of Bojanus {u), which are 

 supposed to excrete from the blood — which passes in 

 great quantities through their walls — urinary products 

 which pass out at the opening marked with a black dot 



