266 MANUAL OF ZOOLOGY sect. 



the edge of the mantle or pallium. The mantle really con- 

 sists of separate halves or lobes corresponding with the 

 valves of the shell, but in the position of rest the two lobes 

 are so closely approximated as to appear simply like a mem- 

 brane uniting the valves. At one end, however, the mantle 

 projects between the valves in the form of two short tubes, 

 one (ex. sph) smooth-walled, the other (/'//. sph) beset with 

 delicate processes or 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 (//) is 

 protruded 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 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 the foot and siphons are withdrawn and the valves tightly 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, 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 boun- 

 dary 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 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 pro- 

 duced in the middle ventral line into the keel-like foot, and 



