LAMP-SHELLS 



439 



in outline an ancient lamp, a fact which has given the popular 

 name to the class. The projecting beak of the lower valve is 

 perforated by a rounded hole from which projects a cylindrical 

 stalk by which the animal is attached, and which occupies the 

 same relative position as the wick of the imaginary lamp. The 

 fixed sedentary habit of this and most other forms is another 

 point of contrast with the bulk of bivalve molluscs, which possess 

 a marked power of 

 locomotion. We shall A. 

 further find that either 

 valve is bilaterally sym- 

 metrical, which is not 

 the case with a shell of 

 cockle, mussel, or the 

 like, while the two valves 



,aTOHACH 



HEAR. 



Fig. 271. — Lamp-Shell {WaZdheimia) 



A, Diagram of structure ; body cut through centre, u. Shell, seen from 

 left side, o, Interior of dorsal valve^ to show "carriage-spring" support 

 for arms. 



of such a mollusc are 

 typically equal in size. 

 There are also great dif- 

 ferences in the minute 

 structure of the shell. 



Upon opening the 

 shell of a Waldheimia, 

 the animal will be 

 found to occupy but a 

 very small proportion 

 of the space within, 



but there are two large coiled " arms ", one on either side ot the 

 mouth, fringed with a double set of tentacles and suggesting the 

 tentacular crown of Plumatella, if one supposes the halves of this 

 to be much elongated and coiled up. The so-called arms are 

 supported by a curved loop, shaped like a carriage-spring and 

 projecting from the upper valve. A mantle -lobe lines either 

 shell. 



In a bivalve mollusc the two halves of the shell are drawn 

 together by adductor muscles and opened by an elastic ligament 

 (p- 2)2P)- In a Lamp - Shell both processes are effected by 

 muscles, and the hinge along which movements take place runs 

 transversely and is placed close to the beak. The gut is a bent 

 tube placed in the middle plane and consisting of gullet, stomach, 

 and intestine, the last ending blindly. A good-sized digestive 



