110 THE STORY OF ANIMAL LIFE 



Something must be -said regarding the interior 

 of the shell. The majority of bivalve shells 

 have a complicated system of so-called " teeth," 

 or interlocking projections, at the hinges of the 

 shell : these exhibit great variety in different 

 kinds of shell, and are therefore often a ready 

 means of distinguishing one shell from another. 

 The Scallop, however, is very deficient in this 

 respect, as are also some of its near relations, 

 for instance the oyster and its family group. The 

 Fresh-water Mussel also gains its name, Anodon, 

 or Anodonta, the Toothless One, from the same 

 circumstance. The name often puzzles the be- 

 ginner, who asks, bewildered, " But do Bivalves 

 ever have any teeth?" True teeth, of course, 

 they have none it is the shell-hinge that has 

 teeth, not the animal inside it. Not only have 

 the bivalve shell-fish no teeth indeed, or tongue- 

 ribbon, but furthermore they have no head. For 

 this reason the group has not only received the 

 name already mentioned, of Aglossa, the Tongue- 

 less Ones, but also that of Lipocephala, i.e. 

 Molluscs in which the head is not developed. 

 The reason of its absence is not far to seek 

 a head would be no use inside such a shell. The 

 snail-shell, so differently built, allows freedom 

 for the head ; the bivalve mollusc, squeezed in 

 between its valves, has room only for a mouth. 



We have referred above to the ridges on the 

 outside of the shell. Now that we have learnt 

 that the Pecten is a very active animal, and moves 

 in the manner described, we see that these ridges 

 run parallel to the direction in which it moves as 

 it darts away ears foremost. Let us try to realise 

 what is the effect of this. 



Take a mat with parallel stripes and move it 



