36 THE SCALLOP FISHERY 



The Bissoconch Stage. 



The young scallop now enters upon the third stage of its development, 

 the period of byssal attachment, which is comparable to youth in mau. 

 From structural differences of shell, which sharply distinguish it from 

 the prodissoconch, it has been called by Professor Jackson (4) the dis- 

 soeonch stage. The anatomical changes are so complicated that for the 

 purpose of description several arbitrary subdivisions, illustrating suc- 

 cessive periods of development, have been made. A table of these 

 phases is appended in chapter YII. In the general description of the 

 dissoconch period, especially in the section on anatomical development, 

 reference is made to these subdivisions. 



The chief characteristics of the dissoconch stage are the habits of 

 byssal fixation and crawling. In a preliminary report the writer was 

 led to include an intermediate stage between the free sw immin g veliger 

 and the attached scallop, that of a free crawling existence. Later in- 

 vestigation has shown that the last two stages practically coincide, and 

 that no line of distinction can be drawn. Evidently the power of 

 crawling is supplementary to byssal fixation, and is of great service 

 to the animal when it wishes to change its location or is torn away 

 from its point of attachment. That young scallops have the power of 

 byssal fixation immediately following the prodissoconch or at the very 

 beginning of the dissoconch stage is shown by those attached to the 

 raft spat boxes, described in chapter VII. In many of these scallops 

 the dissoconch growth, scarcely one day old, had just started, yet they 

 at once attached themselves, by a fine byssal thread, to the sides and 

 bottom of glass dishes. 



The subsequent , changes in anatomy and shell formation can be 

 more readily attributed to a complete change in habits, such as the 

 assuming of a stationary life after a free-swimming existence, than 

 the transition from swimming to' the intermediate crawling stage, such 

 as has been suggested by other investigators. Knowledge of the byssal 

 attachment in the early part of the dissoconch stage shows that there 

 is an abrupt change of life at this period, and gives a new interpreta- 

 tion to the structural differences. 



The Set. — The oyster, according to Jackson (4), still possesses a 

 velum when it " spats," or attaches itself at the end of the prodissoconch 

 stage, and no foot has developed. " The preliminary fixation," he 

 states, "is probably effected by means of the reflected mantle border, 

 as described by Ryder, and is then immediately succeeded by a cement- 

 ing conchyolin attachment at the extreme edge of the lower left prodis- 

 soconch valve." The scallop, on the other hand, before it sets has lost 

 its velum and has developed a muscular foot, which acts as a swimming 

 organ during the latter part of the prodissoconch stage. The set is 

 made, not by any fixation of the shell, but by a fine thread, called the 



