CHAP, viii EXTERNAL CHARACTERS 397 



(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, how- 

 ever, the mantle projects between the valves in the form 

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

 the other (in. sph) 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 (ft) 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 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 mussel is irritated the foot and siphons are withdrawn 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 short 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 



