82 Shell Life 



quently be very powerful. The ribs of the Scallop 

 shell serve another important purpose besides that 

 of combining strength with lightness : at the lower 

 margin the ribs and intervening spaces of the two 

 valves so accurately fit together that with the 

 almost central muscle holding the valves together 

 any shifting of them to right or left is impossible. 

 The development of the "ears" at each side of the 

 beak gives similar strength to the upper margin, 

 and allows of the employment of a long though 

 narrow ligament. The foot is developed as a finger- 

 like organ, occupied in the spinning of byssal threads, 

 and space is left between the ears for the passage 

 of these, the two valves being kept slightly apart 

 at this place by a little boss. Ordinarily the shell 

 is kept slightly open by the pad of elastic cartilage 

 between the beaks, and then the mantle is seen to 

 great advantage. Its margin is double, the inner 

 finely fringed and the outer edged with long 

 tentacles, and at their base gleam a row of large 

 and brilliant opalescent " eyes," whose optical powers 

 appear to be of a relatively high order. All the 

 British species are unattached 

 with the exception of — 



The Hunchback Scallop (P. 

 piisio), which in the adult state 

 becomes attached like an Oyster 

 by its lower valve. Though it 

 begins life with as symmetrical a 

 ^^ , ^ ,, shell as the others, it has a liking 



Hunchback Scallop ' ^ 



for contracted quarters, such as the 

 interior of vacated shells, clusters of certain corallines 

 (Eschara), etc. After attaching the newest portion 



