14 THE SCALLOP FISHERY 



varies from 14 to 19, the average being 16. Crossing the radiating ribs 

 are concentric growth lines, which almost show the daily accretion of the 

 shell. Age and wear cause these to be less conspicuous toward the beak 

 and on the loAver valve. During the winter months growth ceases and 

 when again resumed in the spring, a heavy line has formed by the 

 thickening of the edge of the shell, comparable to the annual ring of a 

 tree. This is the so-called growth line, which defines a " seed " or 

 immature scallop. Such marks may not always represent the end of 

 the season's growth, but may indicate that unfavorable conditions for a 

 certain length of time checked the building processes of the animal. 

 When scallops were transplanted from Inward Point to Monomoy 

 Point, being confined several days, the change caused a mark similar 

 to the annual growth line. 



In old age these growth lines may pile up and form a very slight 

 re-entrant in the shell, due to retrogressive development. The re-entrant 

 is not so conspicuous in Pecten as in other lamellibranchs which have 

 a thicker shell. If the edge of one valve is broken, the opposite valve 

 grows down abnormally to protect the soft parts. Both valves are 

 needed in place to get a normal growth in the same manner as rodents 

 require both sets of incisors to wear on each other. 



Besides these markings the outer surface of the shell may be engraved 

 by various enemies. The oyster drill often punctures a small hole in the 

 shell and through it sucks out the soft parts. The boring sponge does 

 not attack the scallop as frequently as it does the oyster and other 

 mollusks which lead a sedentary life, but occasionally parts of the shell 

 are dissolved by the secretions of the sponge. In the older scallop of 

 fifteen months the upper valve is usually covered with various forms of 

 life, such as Serpula (worm lubes), barnacles, young oysters, Anomia 

 (silver shells), Crepidula (quarter deckers), Acmaea and such sea weeds 

 as Enteromorpha, Ulva and Champi parvia. Many old scallops are 

 doubtless killed by the accumulation of foreign growth, which at times 

 fastens the valves together. 



The inner surface of the shell is very smooth and somewhat vitreous, 

 due to the nacreous or pearly material secreted by the mantle. The 

 ridges and furrows exist but are not conspicuous. The scar, which 

 marks the attachment of the adductor muscle, can be seen indistinctly 

 outlined slightly to one side of the center of the shell. 



The hinge line (HH) is curved in most lamellibranchs, but in Pecten 

 it is straight, and extends to the end of the well-developed " ears." A 

 V-shaped spring in the central part of the hinge tends to keep the 

 valves apart after the same manner of doubling a large piece of rubber 

 in the hinge of a door. When the animal is at rest the large adductor 

 muscle is relaxed and the valves remain open. 



The calcareous shell is formed by the secretions of the mantle. Al- 

 though it is added to slightly by the surface of the mantle, the main 

 increase is in height and width at the edge of the mantle. These two 



