OF MASSACHUSETTS. 39 



The s|Hviti<.' development for each set of organs is given in del ail under 

 the section on "Anatomical Development," and it is only necessary to 

 give here a brief resume of the more important changes. 



As the animal has entered upon an alternately stationary and crawling 

 existence, the foot has become relatively the most important organ, and 

 during the early part of the dissoconch stage reaches its maximum 

 development in size. The ciliated tip and muscular body render it an 

 active organ of locomotion, while the byssus gland provides the scallop 

 with a means of attachment. The mantle, at first a simple, curtain-like 

 fold with ciliated edges, becomes more specialized by the development 

 on its outer edge of a few tentacles and eyes, which give it greater 

 sensory functions. The four folds of the inner gills of the prodis- 

 soconch increase to twenty-two, and the outer gills make their appear- 

 ance before the dissoconch stage is completed. The digestive organs 

 increase in size, the liver becoming the most prominent, while the in- 

 testine elongates so that the anal opening is on the postero-ventral side 

 of the adductor muscle. The posterior adductor muscle, which through 

 this period has been capable of great expansion, as is shown in Fig. 23, 

 so that the shell is often opened to an angle of 90, has grown larger 

 in circumference and has taken a more central position. The heart, 

 consisting of a ventricle and two auricles, with its supplementary cir- 

 culatory system, now first becomes conspicuous (Fig. 27). Altogether 

 the internal anatomy of the young scallop has passed through the tran- 

 sition period from babyhood to the adult, and is now ready to take on 

 the final characteristics of the mature scallop. 



The Plicated Stage. 



The plication stage, as the name suggests, marks the beginning of the 

 radiating ridges or furrows which give to the scallop its beautiful fan- 

 like appearance. These plications do not increase in number as the 

 animal grows older. Figs. 33 and 34 show the beginnings of the plica- 

 tions in the shell, while Figs. 36 and 37 show a later stage. In the early 

 plicated stage the form of the scallop is semi-circular, the height and 

 width being approximately the same, while the hinge line is nearly equal 

 to the width. The hinge line is now straight, but markings exist in the 

 shell showing the former downward slant toward the prodissoconch, 

 which in the early part of the plication period is asymmetrical, but 

 later attains a median position, before it is covered by the rounding 

 umbones of the shell. The true " ears " of the adult make their appear- 

 ance as indentations on the lower sides of the hinge line, anteriorly and 

 posteriorly, when the scallop has attained about 2 millimeters in size. 

 During this stage they are much less pronounced than in the adult, while 

 the hinge line itself is relatively longer, nearly equaling the width of 

 the shell. 



The byssal notch, which inclines slightly upward toward the prodisso- 



