CHAP, vin EXTERNAL CHARACTERS 397 



the shell (compare Fig. 103), 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 projects between the valves in the form of two 

 short tubes, one (Fig. 101, ex. sph) smooth-walled, the 

 other (in. spJi) beset with delicate processes or tentacles. 

 By diffusing particles of carmine or indigo in the water it can 

 be seen that a current is always passing in at the fringed tube 

 -hence called the inhalant siphon^ and out at the smooth or 

 exhalant siphon. Frequently a semi-transparent, tongue-like 

 body (//) is protruded between the valves at the opposite 

 side from the hinge and at the end furthest from the siphons : 

 this is they^tf/, by means of which the animal is able slowly 

 to plough its way through the sand or mud. When the 

 mussel is irritated the foot and siphons are withdrawn 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 corresponding lobe of the 

 mantle, and 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 regions of the valves : it is 

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

 and on either side between the foot and the corresponding 

 mantle-lobe are two delicate, striated plates, the gills or 



