144 INTRODUCTION TO ZOOLOGY CHAP. 



is limited below in the same way. Further dissection and 

 examination is best done under water. 



When the mantle-lobe on one side is removed, 

 a ' the plate-like gills or respiratory organs (Fig. 91, g) 

 will be seen lying right along the middle of the body above 

 the foot, which projects in front but is probably much con- 

 tracted. There are four of these gills, two lying on either 

 side of the foot. They should be pushed aside with a blunt 

 instrument (a seeker) one after the other, so as to expose 

 those lying below. The gills have a spongy, porous texture, 



!rn 



Jo mouth 



FIG. 91. The Swan Mussel. 



aa and pa, Muscles which close shell ; ar, pr, muscles which move shell forwards ; lm, 

 cut edge of the left flap of the mantle ; , anus ; /, foot ; g, gills ; rm, edge of right 

 mantle-flap. 



being formed of a very complicated system of vertical and 

 horizontal bars united into a kind of close trellis-work the 

 vertical bars causing the striation visible on the surface of 

 the gills. 



The impure blood from the foot and viscera is carried by 

 special vessels first to the kidneys, and then either to these 

 gills or to the mantle -lobes. In either case the blood is 

 purified by the oxygen in the water which enters by the 

 inhalent aperture, washes over the inner surfaces of the 

 mantle-lobes, over and through the gills, and finally passes 

 into the cavity above the gills, and out of the exhalent 

 aperture. This water current is maintained by the lashing 

 of the hairs which cover the gills, but which can only be 

 seen under a microscope. The purified blood is carried by 

 special veins to the right and left auricles of the heart, which 



