Molluscs 133 



to lying upon their backs until the more 

 progressive members took action to 

 remedy the various disadvantages in- 

 separable from the supine status. The 

 mantle of the whelk or snail is a clearly 

 visible sheet of fleshy substance for ever 

 exuding a form of mortar which on 

 exposure hardens into carbonate of lime. 

 Many bivalves are sedentary or at 

 best hampered by very limited powers 

 of movement. Their bodies are well 

 developed whilst the muscles which keep 

 the shell open or shut at the tenant's 

 will are very powerful. The " foot " 

 which can enable a big snail to travel at 

 the speed of a quarter of a mile per hour 

 may, in the bivalves, be turned into a 

 plough for furrowing mud, drilling rock, 

 or spinning a number of " anchor ropes." 

 There are, however, many sedentary 

 forms, the majority of which, owing to 

 the loss of the tough locomotory foot, are 

 edible. 



