274 



SWAN MUSSELS. PHYLUM MOLLUSCA 



muscle, which draws the body backward upon the foot. If the 

 mantle' is turned back the rest of the external organs are laid 



bare. 



At the front and in the middle is a wedge-shaped organ called 

 the foot ; its lower part is muscular, and the upper part contains 

 the genital organs and intestine. Blood can be forced into sinuses 

 which it contains and is prevented from returning by sphincter 

 muscles round the veins ; the foot is thus caused to protrude 

 between the valves and to swell, in much the same manner 

 as does the mammahan penis when it is erected. In this state it 

 is wedged into the mud or between the stones at the bottom of the 

 water, and when the retractor muscles contract the body is 



cl.c. 



ax.g. 



Fig. 20I. 



A B C D 



-Diagrams of transverse sections through the swan musseL 



A passes through the middle of the foot and shows the inner lamella of the inner gill attached to the side 

 of the foot ; B passes through the hinder part of the foot and shows the inner lamella of the inner 

 gill free ; C is taken behind the foot and shows the inner lamella of the inner gills joining in the middle 

 line ; D is further back and shows the axes of the gills free. 



ax.g.. Axes of the gills ; cl.c, cloacal chamber ; ep.sp., epibranchial space ; /., foot ; i.g.i, inner lamella of 

 inner gill ; i.g.2, outer lamella of inner gill ; il.sp., interlaraellar space ; o.g.i, inner lamella of outer 

 gill ; o.g.2, outer lamella of outer gill ; ml., mantle lobe ; tnl.c, mantle cavity. 



pulled forwards. The foot is withdrawn by the contraction of its 

 own muscles, which empty the sinuses. Between the foot and the 

 mantle on each side, and extending to the posterior end, are two 

 double flaps called gills, although they have little function as 

 such. At the front end of the foot and also inside the mantle, is 

 another double pair of flaps, the labial palps, which do not in 

 the least resemble the structures of that name in insects. Between 

 the palps lies the mouth ; its upper lip joins the outer palps in 

 front of the mantle, and its lower lip joins the inner palps behind. 

 The structure of the gills is complicated and is shown in 

 Figs. 201, 202. The general structure of each gill is like that of 

 two sheets of trellis work, joined continuously along the bottom 

 edge, and at intervals elsewhere. Posteriorly the inner lamella 

 of the outer gill, and the whole of the inner gill, become free 

 from the body, although the inner lamellae of the two inner gills 



