LESS. XXVIII GENERAL STRUCTURE 349 



the shell, but in the position of rest the two lobes are so 

 closely approximated as to appear simply like a membrane 

 uniting the valves. At one end, however, the mantle pro- 

 jects between the valves in the form of two short tubes, one 

 (Fig. 92, B, ex. sph.) smooth-walled, the other {in. sph.) beset 

 with delicate processes ox fimbria. By diffusing particles of 

 carmine or indigo in the water it can be seen that a current 

 is always passing in at the fimbriated tube, hence called the 

 inhalant siphon, and out at the smooth or exhalant siphon. 

 Frequently a semi-transparent, tongue-like body {Jt) is pro- 

 truded between the valves at the opposite side from the 

 hinge and at the end furthest from the siphons : this is the 

 foot, by its means the animal is able slowly to plough its 

 way through the sand or mud. When irritated the foot and 

 siphons are withdrawn and the valves tightly closed. In a 

 dead animal, on the other hand, the shell always gapes, and 

 it can then be seen that each valve is lined by the corre- 

 sponding lobe of the mantle, that the exhalant siphon is 

 formed by the union of the lobes above and below it and 

 is thus an actual tube, but that the boundary of the inhalant 

 siphon facing the gape of the shell is simply formed by the 

 approximation of the mantle-lobes, so that this tube is a 

 temporary one. 



The hinge of the shell is dorsal, the gape ventral, the end 

 bearing the siphons posterior, the end from which the foot 

 is protruded anterior : hence the valves and mantle-lobes 

 are respectively right and left. 



In a dead and gaping mussel the general disposition of 

 the parts of the animal is readily seen. The main part of 

 the body lies between the dorsal ends of the valves : it is 

 produced in the middle ventral line into the keel-like foot : 

 and on each side, between the foot and the corresponding 

 mantle-lobe, are two delicate, striated plates, the gills. Thus 



