STRUCTURE AIND PHYSIOLOGY OF THE MOLLUSC A. 'A3 



had settled, compelling the animal to contract itself, faster even 

 than it could form shell to fill up the void. 



Lines of growth. So long as the animal continues grooving, 

 each new layer of shell extends beyond the one formed before it ; 

 and, in consequence, the external smface becomes marked with 

 lines of groictJl. During winter, or the season of rest which cor- 

 responds to it, shells cease to grow ; and these periodic resting- 

 places are often indicated by interruptions of the otherwise regu- 

 lar lines of growth and colour, or by still more obvious signs. It 

 is probable that this pause, or cessation from growth, extends 

 into the breeding season ; otherwise there would be two periods 

 of growth, and two of rest in each year. In many shells the 

 gTOwth is uniform ; but in others each stage is finished by the 

 development of a fringe, or ridge (varix), or of a row^ of spines, 

 as in tridacna and murex. {Oioen, Gra)it,) 



Adult characters. The attain- 

 ment of the full-growth proper to 

 each species is usually marked by 

 changes in the shell. 



Some bivalves, like fhe oyster, 

 and gryphcea (fig. 26), continue to 

 increase in thickness long after 

 they have ceased to grow outwards; ^'^- '^^' ^''^''''' ofgryj^hoea* 

 the greatest addition is made to the lower valve, especially near 

 the umbo ; and in the spondylus some parts of the mantle secrete 

 more than others, so that cavities, filled with fluid, are left in the 

 substance of the shell. 



The adult teredo 2Ci\A. fistulana close the end of their burrows ; 

 the pholadidea fills up the great pedal opening of its valves ; and 

 the aspergillum forms the porous disk from which it takes its 

 name. Sculptured shells, particularly ammonites, and species of 

 rostellaria and fusus, often become plain in the last part of their 



* Fig. 26. Section oi gryphofa incurva, Sl)y. Lias, Dorset, (original ; dimi- 

 nished one half), the upper valve is not much thickened ; the interior is filled 

 with lias. 



