Fig. 38. Dorsal view of Pecten in plicated stage, showing umbones and 

 hinge line. The left valve is deeper than the right. The prodissoconch (pd) is 

 sharply marked off, the amount of separation between its two valves being well 

 shown. Magnified 32 diameters. 



Fig. 39. Plicated stage. A 2.15-millimeter scallop magnified 32 diameters. 

 View of corner of right valve, showing groove and notch. Nine teeth (te) can be 

 seen in the byssal area, six of which are within the external border of the groove. 

 There is a second furrow dorsal to the byssal groove, and a serrated structure near 

 the hinge line of seven sharply pointed teeth, which possibly may be an individual 

 variation. 



Fig. 40. Posterior view of scallop in the plicated stage, showing that the 

 shell has become more nearly equivalvular. The plicated (pi) and dissoconch (d) 

 areas are sharply differentiated. Magnified 32 diameters. 



Fig. 41. Prismatic structure of right valve of dissoconch shell highly mag- 

 nified. This structure is not found on the left valve, or on either valve of the adult. 

 Magnified 340 diameters. 



Fig. 42. Foot of 25-millimeter (about 1 inch) scallop, showing cleft (fc), disc- 

 like tip, byssal gland (bg) and groove (fg). Magnified 7 diameters. 



Fig. 43. Byssus of scallop of dissoconch stage after having been cast off by 

 the animal; distal (di) or attached end; proximal (pr) or gland end. Magnified 

 5 diameters. 



Figs. 44-46. Stages showing the development of the tenacle. Fig. 44 rep- 

 resents the first appearance, Fig. 45 further development, and Fig. 46 the tip of 

 a completely formed tenacle. The tenacles on the edge of the mantle are used as 

 sensory, clinging and crawling organs. Magnified 110 diameters. 



Fig. 47. Scallop (4 to 5 millimeters in size) drifting just below the surface 

 of water in aquarium (see page 60). Note the extended tentacles (t), open shell 

 and the reverse position, with right valve uppermost. Magnified 5 diameters. 



Fig. 48. Scallop (1.5 millimeters in size) attached to eel grass by a two- 

 stranded byssus (b), formed during the night. Magnified 7 diameters. 



