40 THE SCALLOP FISHERY 



conch, has now become deeper and is lined with several teeth along its 

 inner border. The number varies from one to six or more, the older 

 teeth being less pointed than the last formed. In Fig. 39 there is a 

 secondary furrow dorsal to the main groove, and a serrated structure 

 near the hinge line, consisting of seven sharply pointed teeth, the origin 

 and use of which are unknown. 



The dorsal view of the shell of Pecten at this stage (Fig. 38) shows 

 the relative size of the umbones and the hinge line. The left valve is 

 deeper than the right and the umbones point slightly posteriorly. The 

 line of separation of the prodissoconch and dissoconch growth is sharply 

 marked, showing how the two valves, which were close together during 

 the prodissoconch stage, have been spread apart by the new growth of 

 the valves. This period is just previous to the disappearance of the 

 prodissoconch, either by the wearing away of the shell or by the growth 

 of the shell. 



The exact duration of the plication stage cannot be given, as the tran- 

 sition to the adult is gradual. Perhaps the end of this period should 

 come when the animal has attained general adult characteristics. If such 

 a definition be taken, the arbitrary size may be assigned as 4 millimeters, 

 for by that time the visceral mass is well defined, completing the adult 

 anatomy of the scallop. Unless some standard were taken, it would 

 be impossible to tell just when the plication stage ceased and adult life 

 began. Another view would have the plication stage followed by a 

 period of youth, and consider that the adult life was not reached until 

 the animal was a year old. This, perhaps, is a better division, although 

 the characteristics of the youth and the adult are practically the same. 



The Internal Anatomy. Few new organs arise during this stage, 

 which is mostly concerned in the development of those already formed. 

 The most prominent feature is the appearance of the visceral mass 

 with the reproductive organs, which are first noticeable at the size of 

 3 millimeters. The visceral mass grows down from the ventral surface 

 of the foot, which becomes relatively smaller with the growth of the 

 animal. 



At the size of 3 millimeters, the mantle has increased by the formation 

 of a set of guard tentacles, which are situated on the perpendicular flap. 

 The eyes have increased until they number sixteen or more on each lobe 

 of the mantle, while the tentacles have correspondingly increased in 

 size and number. The circulatory and the nervous systems have become 

 more complicated, to meet the requirements of the growing animal, 

 which now has acquired the power of swimming by valvular contraction. 

 The digestive system has expanded, the palps becoming ruffled around 

 the mouth, and the intestine elongated in the region of the visceral 

 mass. The adductor muscle has increased greatly in size and can be 

 seen to consist of two distinct portions. By the completion of the stage, 

 the animal has attained all the organs and .characteristics of the adult 

 scallop. 



