Fig. 17. Early veliger swimming with velum extended. Viewed from side. 

 Magnified 150 diameters. 



Fig. 18. Late veliger or prodissoconch. Note change in form of shell, as 

 compared with Fig. 17. This stage marks the end of the embryonic period, as the 

 scallop now forsakes its free swimming life and attaches itself to objects by means 

 of its byssus or " anchor strands." Magnified 150 diameters. 



Figs. 19 to 32, inclusive, cover the next distinct stage of development. This 

 form is called "dissoconch," i.e., double shell. 



Fig. 19. Dissoconch Phase 1. Early dissoconch growth after scallop has "set." 

 View of right or lower valve. Note beginning of byssal or foot notch (bn). Scal- 

 lop is now capable of byssal attachment. Right valve is slightly smaller than left. 

 Magnified 150 diameters. 



Fig. 20. View of left valve of same scallop as in Fig. 19. Anatomy shown 

 through transparent shell. Note increased number of gill filaments. Magnified 

 150 diameters. 



Fig. 21. Dissoconch Phase 2. About two days after "set." View of trans- 

 parent right valve through which the organs are seen. The right valve is less 

 convex than the left, for aid in crawling. The heart (ht) is observed for the first 

 time during this stage. Note the slower growth of the byssal or foot notch, which 

 is one period behind the growth of the rest of the shell. Note also the increase 

 in the number of gill filaments. Magnified 150 diameters. 



Fig. 22. Internal view of shell of scallop of same age as in Fig. 21, showing ten 

 pairs of small teeth (te), which interlock to form a firm hinge. The shell is inequi- 

 valved, i.e., the right valve (upper in illustration) is less concave than the left. 

 Magnified 150 diameters. 



Fig. 23. Internal view of same scallop as in Fig. 21, showing the adductor 

 muscle and foot, the rest of the soft parts having been removed. Scallops of this 

 age often open the shell to an angle of 90, thus illustrating the flexibility of the 

 adductor muscle. Magnified 150 diameters. 



Fig. 24. Dissoconch Phase 3. View of right valve of scallop about four days 

 after "set," showing anatomy. The left valve projects slightly beyond the right, 

 and the hinge line is inclined slightly upward. Note increased number of gill bars. 

 Magnified 150 diameters. 



