58 



Shell Life 



eaked Leda 



O 



Dwarf Leda 



important advance in development consists in the 

 production of the hinder part of the mantle into two 

 long tubes or siphons, which are 

 thrust out beyond the shell and 

 reach up to the water above the 

 mud and sand in which the creature 

 burrows. The purpose of these 

 siphons is to enable the Leda to 

 remain safely buried whilst it gets 

 a constant fresh current of water over its gills, and 

 to bring microscopic food to its mouth. Through the 

 lower of these tubes the fresh supplies pour in, whilst 

 the vitiated current carrying off waste passes out 

 through the upper tube. With a view to the better 

 protection of these siphons the shell has been elon- 

 gated backwards. In the Dwarf Leda (X. injgm<xa) 

 the siphons are not very long, and they are united 

 throughout their length, showing that though the 

 mollusk affects deep water (20 to 86 fathoms) it does 

 not burrow much below the surface of the mud, or 

 indeed remain entirely covered by it. Its tubes may 

 be entirely withdrawn into the shell, which closes all 

 round as do the Nut-shells. The shell is scarcely 

 a quarter of an inch long. The Beaked Leda (X. 

 vvinuta) is a much larger species, and the great 

 change in the shape of the shell shows 

 that the creature is given to burrowing 

 more deeply and to remain buried. 

 The siphons, which are united only for 

 half their length, have become much 

 longer in order to reach the water 

 above, and the shell has become greatly lengthened 

 behind for their partial protection, there being 



I nterior of Beaked 

 Leda-shell 



