38 THE SCALLOP FISHERY 



The dissoconch scallop differs from the adult, in which the two valves 

 are equal, by having the right or lower valve smaller than the left and 

 less convex. This is undoubtedly a direct adaptation to its method of 

 life during this period, the flat lower valve offering ease and assistance 

 in crawling and attachment. Anomia offers an excellent example of 

 the flattening of the lower valve in an attached animal, and the rounding 

 out of the upper. The same is true of Pecten irradians to a slighter 

 extent, as it is not attached so firmly nor for so long a period as Anomia. 



The most interesting feature of the shell formation during this period 

 is the development of the byssal notch and groove in the anterior part 

 of the lower valve. The notch is the name applied to the indentation 

 (Figs. 19-21), while "groove" refers to the hollow formed by the 

 .growth of the notch (Fig. 25). The notch first makes its appearance 

 close to the prodissoconch, indicating that it starts at the time the 

 animal " sets." By the time that phase 5 is reached a tooth-like process 

 has formed on the notch (Fig. 29). These increase to three in number 

 at the end of the dissoconch stage, and go as high as five or more in the 

 plicated period. Similar teeth are found on the byssal notch of scal- 

 lops less than one year old, as new ones are constantly forming, while 

 the old are covered by the growth of the shell. Old scallops rarely 

 have teeth on the byssal notch. If the shell is broken along the byssal 

 groove in an adult scallop an entire ridge of these teeth can be seen 

 where they have been covered by the growth of the shell. The use of 

 these teeth is unknown, except that they are closely associated with the 

 byssus, as is described in chapter IV. under the habit of byssal fixation. 



The name byssal notch is probably derived from the fact that the 

 byssus comes out of the same indentation in the adult. Perhaps at this 

 stage a more appropriate name would be foot groove, as that organ, 

 in crawling or in spinning the byssus, is thrust out of the opening. 

 There is some difference of opinion as to whether the byssus or the 

 foot is the cause of the formation of this notch. Jackson (4) says 

 that it is formed by the folding back of the mantle, resulting in retarded 

 growth in that locality (Fig. 21). Whether the foot or the byssus 

 thread was the cause of this retardation cannot be stated definitely, 

 although probably both are functional. Although the foot appears 

 before the dissoconch stage it is used as a swimming organ. The byssal 

 notch appears immediately after the prodissoconch stage, corresponding 

 with both the byssal attachment and the use of the foot as a crawling 

 organ. Therefore it can safely be concluded that the byssal notch is 

 characteristic of this period, and is formed by the combined action of 

 foot and byssus. 



The Internal Anatomy. With the development of the shell, corre- 

 sponding changes have taken place in the internal anatomy, rendering 

 the scallop better adapted to its new mode of life. Adult characteristics 

 are now manifest, and the animal can be readily recognized as a scallop. 



